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Available-for-Sale Securities and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Available-for-Sale Securities    
Amortized Cost $ 130,260 $ 148,812
Gross Unrealized Gains 0 1
Gross Unrealized Losses 0 0
Estimated Fair Value $ 130,260 148,813
Maturity period for marketable securities    
Maximum contractual maturity period 1 year  
Weighted average maturity 1 month  
U.S. government agencies    
Available-for-Sale Securities    
Amortized Cost $ 9,989 12,428
Gross Unrealized Gains 0 1
Gross Unrealized Losses 0 0
Estimated Fair Value 9,989 12,429
U.S. commercial paper    
Available-for-Sale Securities    
Amortized Cost 68,365 72,065
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 68,365 72,065
Money market funds    
Available-for-Sale Securities    
Amortized Cost 51,906 64,319
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 51,906 $ 64,319