Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Fair Value - Schedule of fair value of financial assets and liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value $ 354,553 $ 99,113
Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 353,303 55,752
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 1,250 43,361
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 353,303 55,752
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 353,303 55,752
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
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 1,250 43,361
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) | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value $ 1,250 $ 43,361