Income Taxes (Summary of Income Before 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 Taxes [Abstract]
United States Operations									$ 135,177	$ 128,072	$ 69,822
Foreign Operations									18,798	21,043	4,017
INCOME BEFORE INCOME TAXES	$ 45,193	$ 34,396	$ 52,516	$ 21,870	$ 38,685	$ 47,926	$ 35,780	$ 26,724	$ 153,975	$ 149,115	$ 73,839