Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 52,204	$ 51,281	$ 43,226	$ 58,750
Schedule of Available-for-sale Securities [Line Items]
Unrealized Gains	3	1
Unrealized Losses	(10)	(12)
Cash and cash equivalents and available-for-sale securities, amortized cost	57,735	54,942
Cash and cash equivalents and available-for-sale securities	57,728	54,931
U. S. treasury debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,502	3,503
Unrealized Gains	0	0
Unrealized Losses	(6)	(5)
Aggregate Fair Value	3,496	3,498
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,003	7,943
Unrealized Gains	3	1
Unrealized Losses	(4)	(7)
Aggregate Fair Value	9,002	7,937
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	45,230	43,496
Cash and cash equivalents, aggregate fair value	45,230	43,496
Cash and cash equivalents | Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	45,230	43,496
Cash and cash equivalents, aggregate fair value	45,230	43,496
Marketable securities | U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,505	11,446
Unrealized Gains	3	1
Unrealized Losses	(10)	(12)
Aggregate Fair Value	$ 12,498	$ 11,435