We recognized share-based payment expense of $84,310, $78,212 and $68,876 for the years ended December 31, 2015, 2014 and 2013, respectively.

We account for equity instruments granted to employees in accordance with ASC 718, Compensation - Stock Compensation.  ASC 718 requires all share-based compensation payments to be recognized in the financial statements based on fair value. ASC 718 requires forfeitures to be estimated at the time of grant and revised in subsequent periods if actual forfeitures differ from initial estimates. We use the Black-Scholes-Merton option-pricing model to value stock option awards and have elected to treat awards with graded vesting as a single award. Share-based compensation expense is recognized ratably over the requisite service period, which is generally the vesting period, net of forfeitures. We measure restricted stock awards and units using the fair market value of the restricted shares of common stock on the day the award is granted.

Fair value as determined using the Black-Scholes-Merton model varies based on assumptions used for the expected life, expected stock price volatility and risk-free interest rates.  For the years ended December 31, 2015, 2014 and 2013, we estimated the fair value of awards granted using the hybrid approach for volatility, which weights observable historical volatility and implied volatility of qualifying actively traded options on our common stock.  The expected life assumption represents the weighted-average period stock-based awards are expected to remain outstanding.  These expected life assumptions are established through a review of historical exercise behavior of stock-based award grants with similar vesting periods.  Where historical patterns do not exist, contractual terms are used.  The risk-free interest rate represents the daily treasury yield curve rate at the grant date based on the closing market bid yields on actively traded U.S. treasury securities in the over-the-counter market for the expected term. Our assumptions may change in future periods.

Stock-based awards granted to employees, non-employees and members of our board of directors include warrants, stock options, stock awards and restricted stock units.

2015 Long-Term Stock Incentive Plan

In May 2015, our stockholders approved the Sirius XM Holdings Inc. 2015 Long-Term Stock Incentive Plan (the “2015 Plan”).  Employees, consultants and members of our board of directors are eligible to receive awards under the 2015 Plan.  The 2015 Plan provides for the grant of stock options, restricted stock awards, restricted stock units and other stock-based awards that the compensation committee of our board of directors deem appropriate.  Vesting and other terms of stock-based awards are set forth in the agreements with the individuals receiving the awards.  Stock-based awards granted under the 2015 Plan are generally subject to a vesting requirement.  Stock options generally expire ten years from the date of grant.  Each restricted stock unit entitles the holder to receive one share of common stock upon vesting.  As of December 31, 2015, 246,778 shares of common stock were available for future grants under the 2015 Plan.

Other Plans

We maintain four other share-based benefit plans — the Sirius XM Radio Inc. 2009 Long-Term Stock Incentive Plan, the XM 2007 Stock Incentive Plan, the Amended and Restated Sirius Satellite Radio 2003 Long-Term Stock Incentive Plan and the XM 1998 Shares Award Plan. No further awards may be made under these plans.  

The following table summarizes the weighted-average assumptions used to compute the fair value of options granted to employees and members of our board of directors:

The following table summarizes the weighted-average assumptions used to compute the fair value of options granted to third parties, other than non-employee members of our board of directors:

There were no options granted to third parties during the years ended December 31, 2014 and 2013.  We do not intend to pay regular dividends on our common stock.  Accordingly, the dividend yield used in the Black-Scholes-Merton option value was zero for all periods.

The following table summarizes stock option activity under our share-based plans for the years ended December 31, 2015, 2014 and 2013:

The weighted average grant date fair value per share of options granted during the years ended December 31, 2015, 2014 and 2013 was $1.11, $1.05 and $1.48, respectively.  The total intrinsic value of stock options exercised during the years ended December 31, 2015, 2014 and 2013 was $117,944, $89,428 and $142,491, respectively.  During the years ended December 31, 2015, 2014 and 2013, the number of net settled shares which were issued as a result of stock option exercises was 17,652, 15,228 and 32,650, respectively.

We recognized share-based payment expense associated with stock options of $70,084, $69,754 and $66,231 for the years ended December 31, 2015, 2014 and 2013, respectively.

The following table summarizes the restricted stock unit and stock award activity under our share-based plans for the years ended December 31, 2015, 2014 and 2013:

The weighted average grant date fair value per share of restricted stock units and stock awards granted during the years ended December 31, 2015, 2014 and 2013 was $3.92, $3.38 and $3.59, respectively.  The total intrinsic value of restricted stock units and stock awards vesting during the years ended December 31, 2015, 2014 and 2013 was $13,720, $4,044 and $605, respectively.  During the years ended December 31, 2015, 2014 and 2013, the number of net settled shares which were issued as a result of restricted stock units and stock awards vesting were 2,088, 732 and 191, respectively.

We recognized share-based payment expense associated with restricted stock units and stock awards of $14,226, $8,458 and $2,645 during the years ended December 31, 2015, 2014 and 2013, respectively.

Total unrecognized compensation costs related to unvested share-based payment awards for stock options and restricted stock units granted to employees, members of our board of directors and third parties at December 31, 2015 and 2014, net of estimated forfeitures, were $261,628 and $162,985, respectively.  The total unrecognized compensation costs at December 31, 2015 are expected to be recognized over a weighted-average period of 3 years.

401(k) Savings Plan

Sirius XM sponsors the Sirius XM Radio Inc. 401(k) Savings Plan (the “Sirius XM Plan”) for eligible employees. The Sirius XM Plan allows eligible employees to voluntarily contribute from 1% to 50% of their pre-tax eligible earnings, subject to certain defined limits. We match 50% of an employee’s voluntary contributions per pay period on the first 6% of an employee’s pre-tax salary up to a maximum of 3% of eligible compensation.  We may also make additional discretionary matching, true-up matching and non-elective contributions to the Sirius XM Plan based on certain conditions.  Employer matching contributions under the Sirius XM Plan vest at a rate of 33.33% for each year of employment and are fully vested after three years of employment for all current and future contributions.  Beginning in January 2014, our cash employer matching contributions were no longer used to purchase shares of our common stock on the open market, unless the employee elects our common stock as their investment option for this contribution.  We recognized $8,144, $5,385 and $4,181 in expense during years ended December 31, 2015, 2014 and 2013, respectively, to the Sirius XM Plan in fulfillment of our matching obligation.

Sirius XM Holdings Inc. Deferred Compensation Plan

In June 2015, we adopted the Sirius XM Holdings Inc. Deferred Compensation Plan (the “DCP”), effective July 1, 2015.  The DCP allows members of our board of directors and certain eligible employees to defer all or a portion of their base salary, cash incentive compensation and/or board of directors’ compensation, as applicable, each plan year starting in 2016.  Pursuant to the terms of the DCP, we may elect to make additional contributions beyond amounts deferred by participants, but we are under no obligation to do so.  We have established a grantor (or “rabbi”) trust to facilitate the payment of our obligations under the DCP.  As of December 31, 2015, there were no balances or amounts associated with the DCP that were recorded in our consolidated financial statements.