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Marketable Securities and Fair Value Measurements - Securities in Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value    
Less Than 12 Months $ 50,578 $ 53,351
12 Months or More 1,500 0
Total 52,078 53,351
Gross Unrealized Losses    
Less Than 12 Months (33) (64)
12 Months or More (3) 0
Total (36) (64)
Certificates of deposit    
Fair Value    
Less Than 12 Months 5,500  
12 Months or More 0  
Total 5,500  
Gross Unrealized Losses    
Less Than 12 Months (1)  
12 Months or More 0  
Total (1)  
Corporate securities    
Fair Value    
Less Than 12 Months 27,254 28,537
12 Months or More 1,500 0
Total 28,754 28,537
Gross Unrealized Losses    
Less Than 12 Months (10) (42)
12 Months or More (3) 0
Total (13) (42)
U.S. Treasury and agency securities    
Fair Value    
Less Than 12 Months 3,482 5,176
12 Months or More 0 0
Total 3,482 5,176
Gross Unrealized Losses    
Less Than 12 Months (12) (14)
12 Months or More 0 0
Total (12) (14)
Commercial paper    
Fair Value    
Less Than 12 Months   8,974
12 Months or More   0
Total   8,974
Gross Unrealized Losses    
Less Than 12 Months   (2)
12 Months or More   0
Total   (2)
Asset-backed securities    
Fair Value    
Less Than 12 Months 14,342 10,664
12 Months or More 0 0
Total 14,342 10,664
Gross Unrealized Losses    
Less Than 12 Months (10) (6)
12 Months or More 0 0
Total $ (10) $ (6)