Income Taxes - Deferred Tax Valuation Allowance (Details) - Valuation Allowance of Deferred Tax Assets - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Beginning balance	$ 76,395	$ 62,605	$ 27,082
Additions	5,332	13,790	35,523
Balance at December 31, 2017 (before impact of Tax Act)	81,727
Reduction in valuation allowance due to rate change from Tax Act	(22,378)
Ending balance	$ 59,349	$ 76,395	$ 62,605