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Available-for-Sale Securities and Fair Value Measurements - Available-for Sale Securities (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Available-for Sale Securities    
Amortized Cost $ 168,826 $ 148,812
Gross Unrealized Gains 0 1
Gross Unrealized Losses (1) 0
Estimated Fair Value $ 168,825 148,813
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 $ 6,006 12,428
Gross Unrealized Gains 0 1
Gross Unrealized Losses (1) 0
Estimated Fair Value 6,005 12,429
U.S. commercial paper    
Available-for Sale Securities    
Amortized Cost 62,878 72,065
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 62,878 72,065
Money market funds    
Available-for Sale Securities    
Amortized Cost 99,942 64,319
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 99,942 $ 64,319