Annual report pursuant to Section 13 and 15(d)

Composition of Certain Financial Statement Captions - 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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Composition of Certain Financial Statement Captions - 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
Dec. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost $ 312,860 $ 386,791
Unrealized Gains 8 6
Unrealized Losses (349) (453)
Fair Value 312,519 386,344
Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 180,021 89,113
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value 180,021 89,113
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 35,881 54,792
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value 35,881 54,792
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 79,760 235,135
Unrealized Gains 8 6
Unrealized Losses (347) (444)
Fair Value 79,421 234,697
U.S. government and government agency debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 17,198 7,751
Unrealized Gains 0 0
Unrealized Losses (2) (9)
Fair Value $ 17,196 $ 7,742