Fair Value of Financial Assets and Liabilities - by type (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Marketable securities:
Amortized Cost	$ 137,861	$ 144,012
Gross Unrealized Gain		14
Gross Unrealized Loss	(101)	(63)
Fair Value	137,760	143,963
Corporate debt securities
Marketable securities:
Amortized Cost	51,533	51,352
Gross Unrealized Loss	(45)	(59)
Fair Value	51,488	51,293
Commercial paper
Marketable securities:
Amortized Cost	16,848	20,463
Fair Value	16,848	20,463
Asset-backed Securities [Member]
Marketable securities:
Amortized Cost	25,988	28,692
Gross Unrealized Gain		6
Gross Unrealized Loss	(26)	(1)
Fair Value	25,962	28,697
U.S. government agency debt securities
Marketable securities:
Amortized Cost	43,492	43,505
Gross Unrealized Gain		8
Gross Unrealized Loss	(30)	(3)
Fair Value	$ 43,462	$ 43,510