Fair Value of Financial Assets and Liabilities - by type (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Marketable securities:
Amortized Cost	$ 137,893	$ 144,012
Gross Unrealized Gain		14
Gross Unrealized Loss	(104)	(63)
Fair Value	137,789	143,963
Corporate debt securities
Marketable securities:
Amortized Cost	38,318	51,352
Gross Unrealized Loss	(25)	(59)
Fair Value	38,293	51,293
Commercial paper
Marketable securities:
Amortized Cost	34,098	20,463
Fair Value	34,098	20,463
Asset-backed Securities [Member]
Marketable securities:
Amortized Cost	25,938	28,692
Gross Unrealized Gain		6
Gross Unrealized Loss	(21)	(1)
Fair Value	25,917	28,697
U.S. government agency debt securities
Marketable securities:
Amortized Cost	39,539	43,505
Gross Unrealized Gain		8
Gross Unrealized Loss	(58)	(3)
Fair Value	$ 39,481	$ 43,510