Fair Value of Financial Assets and Liabilities - by type (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Marketable securities:
Amortized Cost	$ 66,256	$ 82,341
Gross Unrealized Gain	5
Gross Unrealized Loss	(15)	(154)
Fair Value	66,246	82,187
Corporate debt securities
Marketable securities:
Amortized Cost	36,963	46,270
Gross Unrealized Loss	(14)	(125)
Fair Value	$ 36,949	$ 46,145
Corporate debt securities | Maximum
Marketable securities:
Remaining term of maturities (in months)	12 months	15 months
Commercial paper
Marketable securities:
Amortized Cost	$ 11,975	$ 9,789
Fair Value	11,975	9,789
Asset-backed Securities [Member]
Marketable securities:
Amortized Cost	8,299	6,234
Gross Unrealized Gain	2
Gross Unrealized Loss		(14)
Fair Value	8,301	6,220
U.S. government agency debt securities
Marketable securities:
Amortized Cost	9,019	20,048
Gross Unrealized Gain	3
Gross Unrealized Loss	(1)	(15)
Fair Value	$ 9,021	$ 20,033