3. Composition of Certain Financial Statement Captions

Cash, Cash Equivalents and Investments

        Our short-term investments have maturities of less than twelve months and are classified as available-for-sale. Our long-term investments have maturities of greater than twelve months and are classified as available-for-sale. We did not hold any long-term investments as of January 31, 2013.

        The following table summarizes our available-for-sale securities' adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category as of January 31, 2013 and December 31, 2013.

        The following table presents available-for-sale investments by contractual maturity date as of December 31, 2013. As of January 31, 2013, all of our investments had maturities of less than twelve months.

        Our investment policy requires investments to be investment grade, primarily rated "A1" by Standard & Poor's or "P1" by Moody's or better for short-term investments and rated "A" by Standard & Poor's or "A2" by Moody's or better for long-term investments, with the objective of minimizing the potential risk of principal loss. In addition, the investment policy limits the amount of credit exposure to any one issuer.

        The unrealized losses on our available-for-sale securities as of December 31, 2013 were primarily a result of unfavorable changes in interest rates subsequent to the initial purchase of these securities. As of December 31, 2013, we owned 68 securities that were in an unrealized loss position. We do not intend nor expect to need to sell these securities before recovering the associated unrealized losses.

We expect to recover the full carrying value of these securities. As a result, no portion of the unrealized losses at December 31, 2013 is deemed to be other-than-temporary and the unrealized losses are not deemed to be credit losses. No available-for-sale securities have been in an unrealized loss position for twelve months or more. When evaluating the investments for other-than-temporary impairment, we review factors such as the length of time and extent to which fair value has been below cost basis, the financial condition of the issuer and any changes thereto, and our intent to sell, or whether it is more likely than not we will be required to sell, the investment before recovery of the investment's amortized cost basis. During the eleven months ended December 31, 2013, we did not recognize any impairment charges.

Accounts Receivable

        The following table summarizes our beginning allowance for doubtful accounts balance for each period, additions, write-offs net of recoveries and the balance at the end of each period for the twelve months ended January 31, 2012 and 2013 and the eleven months ended December 31, 2013:

Property and Equipment

        Depreciation and amortization expenses totaled $4.5 million and $7.1 million for the twelve months ended January 31, 2012 and 2013 and $10.1 million for the eleven months ended December 31, 2013. We wrote off net assets due to asset retirement totaling $0.3 million for the twelve months ended January 31, 2012. There were no material write-offs during the twelve months ended January 31, 2013 or the eleven months ended December 31, 2013.

        Software developed for internal use generally has an expected useful life of three years from the date placed in service. As of January 31, 2013 and December 31, 2013 the net carrying amount was $1.0 million and $1.5, including accumulated amortization of $0.1 million and $0.7 million. Amortization expense for the twelve months ended January 31, 2013 and the eleven months ended December 31, 2013 was $0.1 million and $0.6 million. We held no material software developed for internal use assets during the twelve months ended January 31, 2012.