Annual report pursuant to Section 13 and 15(d)

Cash, Cash Equivalents and Investments - Cash Equivalents and Investments (Details)

v3.8.0.1
Cash, Cash Equivalents and Investments - Cash Equivalents and Investments (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 45,494
Unrealized gains 1
Unrealized losses (820)
Fair Value 44,673
U.S. treasuries  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost 4,287
Unrealized gains 0
Unrealized losses (115)
Fair Value $ 4,172
U.S. treasuries | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 212 days
U.S. treasuries | Maximum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 340 days
Asset-backed securities  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 2,709
Unrealized gains 0
Unrealized losses (14)
Fair Value $ 2,695
Asset-backed securities | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 197 days
Asset-backed securities | Maximum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 365 days
Asset-backed securities  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 3,798
Unrealized gains 0
Unrealized losses (110)
Fair Value $ 3,688
Asset-backed securities | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 365 days
Corporate bonds  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 28,356
Unrealized gains 0
Unrealized losses (473)
Fair Value $ 27,882
Corporate bonds | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 91 days
Corporate bonds | Maximum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 365 days
Corporate bonds  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 6,344
Unrealized gains 1
Unrealized losses (108)
Fair Value $ 6,236
Corporate bonds | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 365 days