Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 993,184	$ 953,071	$ 927,859	$ 954,809	$ 884,627	$ 838,883	$ 815,644	$ 844,554	$ 3,828,923	$ 3,383,708	$ 3,207,716
Gross Profit	444,979	406,541	396,763	398,703	383,574	356,743	352,639	365,920
Earnings Before Income Taxes	82,057	53,415	49,980	66,752	60,738	39,221	56,995	82,623	252,204	239,577	218,422
Net Earnings	$ 61,743	$ 43,720	$ 38,501	$ 52,246	$ 177,560	$ 25,341	$ 36,335	$ 53,300	$ 196,210	$ 292,536	$ 139,283
Earnings Per Share (in dollars per share)	$ 0.91	$ 0.64	$ 0.55	$ 0.75	$ 2.51	$ 0.36	$ 0.51	$ 0.75	$ 2.84	$ 4.13	$ 1.93
Earnings Per Share Assuming Dilution (in dollars per share)	$ 0.89	$ 0.62	$ 0.54	$ 0.73	$ 2.46	$ 0.35	$ 0.51	$ 0.74	$ 2.78	$ 4.06	$ 1.91