Fair Value of Financial Assets and Liabilities - by type (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Marketable securities:
Amortized Cost	$ 144,012	$ 82,341
Gross Unrealized Gain	14
Gross Unrealized Loss	(63)	(154)
Fair Value	143,963	82,187
Corporate debt securities
Marketable securities:
Amortized Cost	51,352	46,270
Gross Unrealized Loss	(59)	(125)
Fair Value	51,293	46,145
Commercial paper
Marketable securities:
Amortized Cost	20,463	9,789
Fair Value	20,463	9,789
Asset-backed Securities [Member]
Marketable securities:
Amortized Cost	28,692	6,234
Gross Unrealized Gain	6
Gross Unrealized Loss	(1)	(14)
Fair Value	28,697	6,220
U.S. government agency debt securities
Marketable securities:
Amortized Cost	43,505	20,048
Gross Unrealized Gain	8
Gross Unrealized Loss	(3)	(15)
Fair Value	$ 43,510	$ 20,033