Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents and Investments - Summary of available-for-sale securities' adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category (Details)

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Cash, Cash Equivalents and Investments - Summary of available-for-sale securities' adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost $ 362,626 $ 386,791
Unrealized Gains 37 6
Unrealized Losses (209) (453)
Fair Value 362,454 386,344
Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 94,948 89,113
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value 94,948 89,113
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 50,331 54,792
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value 50,331 54,792
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 214,847 235,135
Unrealized Gains 35 6
Unrealized Losses (209) (444)
Fair Value 214,673 234,697
U.S. government and government agency debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 2,500 7,751
Unrealized Gains 2 0
Unrealized Losses 0 (9)
Fair Value $ 2,502 $ 7,742