Quarterly Financial Data (Unaudited) (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
Provisional one-time charge on US tax reform									$ 3,597	$ 71,982	$ 0
Cash Tax Portion of the One Time Provisional Transition Tax Charge	$ 62,000								62,000
Revenues	817,923	$ 734,846	$ 721,996	$ 660,821	$ 778,031	$ 698,799	$ 653,656	$ 594,567	2,935,586	2,725,053	2,508,257
Gross profit	477,566	419,254	412,625	374,933	455,775	400,975	375,612	343,389	1,684,378	1,575,751	1,437,732
Net earnings	$ 181,186	$ 126,653	$ 111,468	$ 93,304	$ 76,976	$ 104,950	$ 101,580	$ 92,466	$ 512,611	$ 375,972	$ 384,370
Basic earnings per common share:
Net earnings	$ 7.25	$ 5.04	$ 4.41	$ 3.66	$ 3.01	$ 4.10	$ 3.94	$ 3.57	$ 20.33	$ 14.62	$ 14.49
Weighted average number of common shares	24,975,303	25,126,061	25,299,414	25,468,323	25,562,542	25,613,433	25,751,374	25,932,112	25,215,674	25,713,575	26,517,768
Diluted earnings per common share:
Net earnings	$ 7.11	$ 4.93	$ 4.31	$ 3.58	$ 2.93	$ 3.99	$ 3.84	$ 3.48	$ 19.88	$ 14.24	$ 14.22
Weighted average number of common and common equivalent shares	25,490,270	25,683,365	25,867,383	26,095,647	26,229,052	26,303,529	26,439,529	26,586,061	25,781,324	26,393,783	27,023,905
Market price per share:
Common Stock Value During The Period Maximum	$ 645.33	$ 617.94	$ 595.14	$ 692.30	$ 689.11	$ 635.17	$ 601.16	$ 486.90
Common Stock Value During The Period Mimimum	$ 524.03	$ 559.44	$ 546.43	$ 566.87	$ 606.80	$ 571.25	$ 473.87	$ 414.52