Income Taxes - Change In Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance for current year increase in net operating losses	$ 19,378
Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses	11,676
Increase in valuation allowance as a result of foreign currency fluctuation	(541)
Decrease in valuation allowance due to change in tax rates	(43,214)
Net decrease (increase) in valuation allowance	(11,619)	$ 27,273	$ 20,794
U.S.
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance for current year increase in net operating losses	19,117
Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses	11,930
Decrease in valuation allowance due to change in tax rates	(43,214)
Net decrease (increase) in valuation allowance	(12,167)	23,147	22,582
Foreign
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance for current year increase in net operating losses	261
Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses	(254)
Increase in valuation allowance as a result of foreign currency fluctuation	(541)
Net decrease (increase) in valuation allowance	$ 548	$ 4,126	$ (1,788)