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, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost $ 99,165 $ 312,860
Unrealized Gains 3 8
Unrealized Losses (55) (349)
Fair Value 99,113 312,519
Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 55,752 180,021
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value 55,752 180,021
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost   35,881
Unrealized Gains   0
Unrealized Losses   0
Fair Value   35,881
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost 43,413 79,760
Unrealized Gains 3 8
Unrealized Losses (55) (347)
Fair Value $ 43,361 79,421
U.S. government and government agency debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Adjusted Cost   17,198
Unrealized Gains   0
Unrealized Losses   (2)
Fair Value   $ 17,196