Income Taxes (Income Tax Expense (Benefit)) (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
Current income taxes:
Federal									$ 54,977	$ 98,272	$ 62,584
State									12,852	20,041	9,417
Foreign									1,243	(10)	(39)
Total current income taxes									69,072	118,303	71,962
Deferred income taxes:
Federal									51,668	4,464	15,550
State									10,403	(14)	5,962
Foreign									1,172	2,226	1,542
Total deferred income taxes									63,243	6,676	23,054
Total income tax expense	$ 27,004	$ 38,622	$ 37,049	$ 29,640	$ 33,724	$ 31,939	$ 29,930	$ 29,386	$ 132,315	$ 124,979	$ 95,016