Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Financial assets, fair value	$ 46,406	$ 59,588
Money market funds
Financial Assets
Financial assets, fair value	1,674	4,584
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	7,250	39,250
U.S. treasury securities
Financial Assets
Financial assets, fair value	1,501
U.S. government securities
Financial Assets
Financial assets, fair value	35,981	15,754
Level 1
Financial Assets
Financial assets, fair value	3,175	4,584
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	1,674	4,584
Level 1 | U.S. treasury securities
Financial Assets
Financial assets, fair value	1,501
Fair Value, Inputs, Level 2
Financial Assets
Financial assets, fair value	43,231	55,004
Fair Value, Inputs, Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	7,250	39,250
Fair Value, Inputs, Level 2 | U.S. government securities
Financial Assets
Financial assets, fair value	$ 35,981	$ 15,754