Annual report pursuant to Section 13 and 15(d)

Fair Value - Schedule of fair value of financial assets and liabilities (Details)

v3.3.1.900
Fair Value - Schedule of fair value of financial assets and liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value $ 312,519 $ 386,344
Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 180,021 89,113
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 35,881 54,792
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 79,421 234,697
U.S. government and government agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 17,196 7,742
Quoted Prices in Active Markets for Identical Instruments (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 180,021 89,113
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 180,021 89,113
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 0 0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 0 0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | U.S. government and government agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 0 0
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 132,498 297,231
Significant Other Observable Inputs (Level 2) | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 0 0
Significant Other Observable Inputs (Level 2) | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 35,881 54,792
Significant Other Observable Inputs (Level 2) | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 79,421 234,697
Significant Other Observable Inputs (Level 2) | U.S. government and government agency debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value $ 17,196 $ 7,742