Income Taxes (Schedule of Components of Earnings before Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States									$ 213,967	$ 196,827	$ 113,280
Foreign									(1,123)	2,345	1,089
Income before income taxes	$ 43,910	$ 47,616	$ 62,626	$ 58,692	$ 40,203	$ 44,742	$ 58,975	$ 55,252	$ 212,844	$ 199,172	$ 114,369