Income Taxes - Changes in Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance of unrecognized tax benefits	$ 5,632	$ 2,298	$ 1,853
(Decrease) increase for prior year tax positions	(2,466)	1,991	(250)
Increase for current year tax positions	1,255	1,343	695
Total	$ 4,421	$ 5,632	$ 2,298