Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 42,842	$ 39,885	$ 58,750	$ 69,094	$ 86,432	$ 101,131
Schedule of Available-for-sale Securities [Line Items]
Unrealized Gains	13		1
Unrealized Losses	(1)		(65)
Cash and cash equivalents and available-for-sale securities, amortized cost	48,438		70,849
Cash and cash equivalents and available-for-sale securities	48,450		70,785
U. S. treasury debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,505		4,805
Unrealized Gains	1		0
Unrealized Losses	0		(31)
Aggregate Fair Value	3,506		4,774
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,478		12,679
Unrealized Gains	12		1
Unrealized Losses	(1)		(34)
Aggregate Fair Value	7,489		12,646
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	37,455		53,365
Cash and cash equivalents, aggregate fair value	37,455		53,365
Cash and cash equivalents | Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	37,455		53,365
Cash and cash equivalents, aggregate fair value	37,455		53,365
Marketable securities | U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,983		17,484
Unrealized Gains	13		1
Unrealized Losses	(1)		(65)
Aggregate Fair Value	$ 10,995		$ 17,420