Quarterly report pursuant to Section 13 or 15(d)

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

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Cash, Cash Equivalents and Investments - Cash Equivalents and Investments (Details)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 53,993
Unrealized gains 13
Unrealized losses (1,363)
Fair Value 52,643
U.S. treasuries  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost 4,373
Unrealized gains 0
Unrealized losses (198)
Fair Value $ 4,175
U.S. treasuries | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 122 days
U.S. treasuries | Maximum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 250 days
Asset-backed securities  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 5,682
Unrealized gains 0
Unrealized losses (159)
Fair Value $ 5,523
Asset-backed securities | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 107 days
Asset-backed securities | Maximum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 292 days
Asset-backed securities  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 1,879
Unrealized gains 0
Unrealized losses (92)
Fair Value $ 1,787
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 $ 41,257
Unrealized gains 13
Unrealized losses (908)
Fair Value $ 40,362
Corporate bonds | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 25 days
Corporate bonds | Maximum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 356 days
Corporate bonds  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Amortized Cost $ 802
Unrealized gains 0
Unrealized losses (6)
Fair Value $ 796
Corporate bonds | Minimum  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Contractual maturity 365 days