Quarterly Financial Data (Unaudited) Quarterly Financial Data - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Revenues	$ 937,985	$ 807,357	$ 690,673	$ 649,162	$ 843,968	$ 753,866	$ 731,366	$ 679,452
Gross profit	559,044	469,608	397,970	374,409	498,298	435,056	419,538	388,319
Net earnings	$ 216,295	$ 161,767	$ 126,562	$ 98,115	$ 192,749	$ 129,395	$ 127,160	$ 111,805
Basic earnings per common share:
Net earnings	$ 9.15	$ 6.76	$ 5.29	$ 4.08	$ 7.95	$ 5.28	$ 5.15	$ 4.50
Weighted average number of common shares	23,642,415	23,922,272	23,940,278	24,027,833	24,241,383	24,487,268	24,698,032	24,851,340
Diluted earnings per common share:
Net earnings	$ 9.03	$ 6.68	$ 5.22	$ 4.03	$ 7.84	$ 5.20	$ 5.06	$ 4.42
Weighted average number of common and common equivalent shares	23,965,853	24,225,204	24,228,989	24,353,477	24,599,702	24,880,179	25,118,352	25,310,525
Market price per share:
Common Stock Value During The Period Maximum	$ 1,193.36	$ 1,005.15	$ 859.54	$ 835.13	$ 793.31	$ 872.45	$ 840.00	$ 729.72
Common Stock Value During The Period Mimimum	$ 966.92	$ 804.79	$ 630.58	$ 589.41	$ 658.61	$ 642.84	$ 701.80	$ 513.56