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Marketable Securities and Fair Value Measurements - Estimate of Fair Value of Marketable Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 83,991 $ 85,417
Gross Unrealized Gains 18 19
Gross Unrealized Losses (36) (64)
Fair Value 83,973 85,372
Certificates of deposit    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 15,990 12,499
Gross Unrealized Gains 4 9
Gross Unrealized Losses (1) 0
Fair Value 15,993 12,508
Corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 40,941 42,765
Gross Unrealized Gains 12 9
Gross Unrealized Losses (13) (42)
Fair Value 40,940 42,732
U.S. Treasury and agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,494 5,190
Gross Unrealized Gains 0 0
Gross Unrealized Losses (12) (14)
Fair Value 3,482 5,176
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 9,214 11,470
Gross Unrealized Gains 2 1
Gross Unrealized Losses 0 (2)
Fair Value 9,216 11,469
Asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 14,352 13,493
Gross Unrealized Gains 0 0
Gross Unrealized Losses (10) (6)
Fair Value $ 14,342 $ 13,487