Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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 Information Disclosure [Abstract]
Total revenues	$ 2,920,405	$ 3,012,292	$ 2,672,195	$ 2,518,829	$ 2,673,632	$ 2,823,176	$ 2,782,449	$ 2,608,355	$ 11,123,721	$ 10,887,612	$ 10,632,505
Gross profit	425,675	431,420	404,892	383,522	389,181	406,668	410,119	389,101	1,645,509	1,595,069	$ 1,533,972
Net income	$ 110,489	$ 29,881	$ 39,133	$ 33,939	$ 30,828	$ 35,366	$ 46,580	$ 34,291	$ 213,442	$ 147,065
Basic earnings per common share	$ 5.27	$ 1.43	$ 1.84	$ 1.58	$ 1.44	$ 1.65	$ 2.12	$ 1.47	$ 10.08	$ 6.67	$ 3.91
Diluted earnings per common share	$ 5.27	$ 1.43	$ 1.84	$ 1.58	$ 1.44	$ 1.65	$ 2.12	$ 1.47	$ 10.08	$ 6.67	$ 3.90