Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 39,885	$ 43,226	$ 58,750	$ 86,432	$ 89,893	$ 101,131
Schedule of Available-for-sale Securities [Line Items]
Unrealized Gains	25		1
Unrealized Losses	(1)		(65)
Cash and cash equivalents and available-for-sale securities, amortized cost	45,687		70,849
Cash and cash equivalents and available-for-sale securities	45,711		70,785
U. S. treasury debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,507		4,805
Unrealized Gains	6		0
Unrealized Losses	0		(31)
Aggregate Fair Value	3,513		4,774
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,513		12,679
Unrealized Gains	19		1
Unrealized Losses	(1)		(34)
Aggregate Fair Value	7,531		12,646
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	34,667		53,365
Cash and cash equivalents, aggregate fair value	34,667		53,365
Cash and cash equivalents | Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	34,667		53,365
Cash and cash equivalents, aggregate fair value	34,667		53,365
Marketable securities | U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,020		17,484
Unrealized Gains	25		1
Unrealized Losses	(1)		(65)
Aggregate Fair Value	$ 11,044		$ 17,420