Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financial Assets
Financial assets, fair value	$ 55,098	$ 59,588
Money market funds
Financial Assets
Financial assets, fair value	3,781	4,584
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	10,250	39,250
US government and agency securities
Financial Assets
Financial assets, fair value	41,067	15,754
Level 1
Financial Assets
Financial assets, fair value	3,781	4,584
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	3,781	4,584
Level 2
Financial Assets
Financial assets, fair value	51,317	55,004
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	10,250	39,250
Level 2 | US government and agency securities
Financial Assets
Financial assets, fair value	$ 41,067	$ 15,754