Income Taxes - Components of Income before Income Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Income Before Income Tax [Line Items]
Income Before Income Tax Expense	$ 383,896	$ 226,831	$ 119,893
United States [Member]
Schedule Of Income Before Income Tax [Line Items]
Income Before Income Tax Expense	379,407	204,920	81,157
Non-US [Member]
Schedule Of Income Before Income Tax [Line Items]
Income Before Income Tax Expense	$ 4,489	$ 21,911	$ 38,736