Summary of Quarterly Financial Data (Unaudited) (Narrative) (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
Quarterly Financial Data [Abstract]
Loss on early extinguishment of debt	$ 23,200								$ 23,160	$ 4,735	$ 1,533
Income Tax Contingency [Line Items]
Income tax expense	74,361	$ 54,668	$ 41,788	$ 33,847	$ 39,519	$ 37,375	$ 33,574	$ 31,199	$ 204,664	$ 141,667	$ 132,491
Federal
Income Tax Contingency [Line Items]
Income tax expense	$ 47,200