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Available-for-Sale Securities and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Available-for-Sale Securities    
Amortized Cost $ 162,583 $ 148,812
Gross Unrealized Gains 0 1
Gross Unrealized Losses (3) 0
Estimated Fair Value $ 162,580 148,813
Maturity period for marketable securities    
Maximum contractual maturity period 1 year  
Weighted average maturity 4 months  
U.S. government agencies    
Available-for-Sale Securities    
Amortized Cost $ 9,959 12,428
Gross Unrealized Gains 0 1
Gross Unrealized Losses 0 0
Estimated Fair Value 9,959 12,429
U.S. corporate notes    
Available-for-Sale Securities    
Amortized Cost 9,879  
Gross Unrealized Gains 0  
Gross Unrealized Losses (3)  
Estimated Fair Value 9,876  
U.S. commercial paper    
Available-for-Sale Securities    
Amortized Cost 24,582 72,065
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 24,582 72,065
Money market funds    
Available-for-Sale Securities    
Amortized Cost 118,163 64,319
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 118,163 $ 64,319