Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financial Assets
Financial assets, fair value	$ 49,206	$ 59,588
Money market funds
Financial Assets
Financial assets, fair value	5,497	4,584
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	7,250	39,250
US government and agency securities
Financial Assets
Financial assets, fair value	36,459	15,754
Level 1
Financial Assets
Financial assets, fair value	5,497	4,584
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	5,497	4,584
Level 2
Financial Assets
Financial assets, fair value	43,709	55,004
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	7,250	39,250
Level 2 | US government and agency securities
Financial Assets
Financial assets, fair value	$ 36,459	$ 15,754