Quarterly Financial Information (Unaudited) - Quarterly Financial Information (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]
Revenues	$ 884,627	$ 838,883	$ 815,644	$ 844,554	$ 794,954	$ 768,982	$ 789,353	$ 854,427	$ 3,383,708	$ 3,207,716	$ 3,179,756
Gross Profit	383,574	356,743	352,639	365,920	339,599	332,487	352,576	374,268
Earnings Before Income Taxes	60,738	39,221	56,995	82,623	32,288	45,282	61,124	79,728	239,577	218,422	213,120
Net Earnings	$ 177,560	$ 25,341	$ 36,335	$ 53,300	$ 21,631	$ 29,464	$ 38,501	$ 49,687	$ 292,536	$ 139,283	$ 135,709
Earnings Per Share (in dollars per share)	$ 2.51	$ 0.36	$ 0.51	$ 0.75	$ 0.30	$ 0.41	$ 0.53	$ 0.68	$ 4.13	$ 1.93	$ 1.87
Earnings Per Share Assuming Dilution (in dollars per share)	$ 2.46	$ 0.35	$ 0.51	$ 0.74	$ 0.30	$ 0.40	$ 0.53	$ 0.68	$ 4.06	$ 1.91	$ 1.86