Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 43,226	$ 58,750	$ 89,893	$ 101,131
Schedule of Available-for-sale Securities [Line Items]
Unrealized Gains	21	1
Unrealized Losses	(9)	(65)
Cash and cash equivalents and available-for-sale securities, amortized cost	50,072	70,849
Cash and cash equivalents and available-for-sale securities	50,084	70,785
U. S. treasury debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,008	4,805
Unrealized Gains	2	0
Unrealized Losses	(3)	(31)
Aggregate Fair Value	4,007	4,774
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,645	12,679
Unrealized Gains	19	1
Unrealized Losses	(6)	(34)
Aggregate Fair Value	8,658	12,646
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	37,419	53,365
Cash and cash equivalents, aggregate fair value	37,419	53,365
Cash and cash equivalents | Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	37,419	53,365
Cash and cash equivalents, aggregate fair value	37,419	53,365
Marketable securities | U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,653	17,484
Unrealized Gains	21	1
Unrealized Losses	(9)	(65)
Aggregate Fair Value	$ 12,665	$ 17,420