Note 15 - Quarterly Financial Information (Unaudited) - Operating Results by Quarter (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues	$ 2,450,323	$ 2,363,660	$ 2,261,647	$ 2,089,627	$ 2,317,842	$ 2,209,760	$ 2,139,027	$ 1,948,245	$ 9,165,258	$ 8,614,874	$ 7,189,568
Operating income	205,074	167,862	193,093	167,795	122,740	174,688	214,812	168,781	733,825	681,021	623,789
Net earnings	$ 144,676	$ 118,410	$ 133,633	$ 119,601	$ 88,681	$ 131,110	$ 151,652	$ 118,142	$ 516,320	$ 489,585	$ 686,263
Basic earnings per share (in dollars per share)	$ 1.36	$ 1.11	$ 1.24	$ 1.10	$ 0.81	$ 1.20	$ 1.39	$ 1.08	$ 4.81	$ 4.48	$ 6.24
Diluted earnings per share (in dollars per share)	$ 1.35	$ 1.10	$ 1.23	$ 1.09	$ 0.81	$ 1.19	$ 1.37	$ 1.07	$ 4.77	$ 4.43	$ 6.18