Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 68,653	$ 62,040
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	70,502	36,414
Fair Value	68,653	62,040
Total	139,155	98,454
Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	70,502	36,414
Fair Value	68,653	62,040
Total	139,155	98,454
Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Fair Value	0	0
Total	0	0
Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Fair Value	0	0
Total	$ 0	$ 0