Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,672,602	$ 2,800,569	$ 2,726,480	$ 2,432,854	$ 2,538,940	$ 2,626,448	$ 2,511,638	$ 2,260,863	$ 10,632,505	$ 9,937,889	$ 8,918,581
Gross profit	380,133	398,382	391,573	363,884	365,959	374,709	369,148	338,122	1,533,972	1,447,938	$ 1,292,546
Net income	$ (33,387)	$ 45,261	$ 46,310	$ 35,815	$ 18,677	$ 26,162	$ 16,862	$ 31,303	$ 93,999	$ 93,004
Basic earnings per common share	$ (1.41)	$ 1.88	$ 1.91	$ 1.47	$ 0.77	$ 1.07	$ 0.70	$ 1.29	$ 3.91	$ 3.82	$ 4.72
Diluted earnings per common share	$ (1.41)	$ 1.88	$ 1.91	$ 1.47	$ 0.77	$ 1.03	$ 0.62	$ 1.19	$ 3.90	$ 3.60	$ 4.32