Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial Assets
Financial assets, fair value	$ 59,588	$ 24,006
Money market funds
Financial Assets
Financial assets, fair value	4,584	3,988
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	39,250	16,250
US government and agency securities
Financial Assets
Financial assets, fair value	15,754	3,768
Level 1
Financial Assets
Financial assets, fair value	4,584	3,988
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	4,584	3,988
Level 2
Financial Assets
Financial assets, fair value	55,004	20,018
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	39,250	16,250
Level 2 | US government and agency securities
Financial Assets
Financial assets, fair value	$ 15,754	$ 3,768