Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Liabilities:
Payment for contingent consideration	$ 500	$ 800
Decrease in contingent consideration related to expiration of contingent consideration period	1,300
Level 1 | Fair Value, Measurements, Recurring
Assets:
Short-term investments	10,997	42,942
Total	10,997	42,942
Level 3 | Fair Value, Measurements, Recurring
Liabilities:
Contingent consideration	412	2,100
Total	$ 412	$ 2,100