Income Taxes - Uncertain Tax Positions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Gross unrecognized tax benefits
Unrecognized tax benefits at the beginning of the period	$ 41,794	$ 23,254
Gross increase in tax positions for prior year		(51)
Gross increase in tax positions for current year	11,295	18,591
Gross decrease in tax positions for prior years	(685)
Unrecognized tax benefits at the end of the period	$ 52,404	$ 41,794