Fair Value of Financial Assets and Liabilities - by type (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Marketable securities:
Amortized Cost	$ 62,193	$ 82,341
Gross Unrealized Gain	14
Gross Unrealized Loss	(12)	(154)
Fair Value	62,195	82,187
Corporate debt securities
Marketable securities:
Amortized Cost	34,352	46,270
Gross Unrealized Gain	6
Gross Unrealized Loss	(10)	(125)
Fair Value	$ 34,348	$ 46,145
Corporate debt securities | Maximum
Marketable securities:
Remaining term of maturities (in months)	12 months	15 months
Commercial paper
Marketable securities:
Amortized Cost	$ 4,477	$ 9,789
Fair Value	4,477	9,789
Asset-backed Securities [Member]
Marketable securities:
Amortized Cost	8,329	6,234
Gross Unrealized Gain	1
Gross Unrealized Loss	(2)	(14)
Fair Value	8,328	6,220
U.S. government agency debt securities
Marketable securities:
Amortized Cost	15,035	20,048
Gross Unrealized Gain	7
Gross Unrealized Loss		(15)
Fair Value	$ 15,042	$ 20,033