Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Assets
Financial assets, fair value	$ 53,796	$ 59,588
Money market funds
Financial Assets
Financial assets, fair value	3,959	4,584
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	7,250	39,250
U.S. government securities
Financial Assets
Financial assets, fair value	40,080	15,754
U.S. treasury securities
Financial Assets
Financial assets, fair value	2,507
Level 1
Financial Assets
Financial assets, fair value	6,466	4,584
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	3,959	4,584
Level 1 | U.S. treasury securities
Financial Assets
Financial assets, fair value	2,507
Level 2
Financial Assets
Financial assets, fair value	47,330	55,004
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	7,250	39,250
Level 2 | U.S. government securities
Financial Assets
Financial assets, fair value	$ 40,080	$ 15,754