Note 14 - Quarterly Financial Information (Unaudited) - Operating Results by Quarter (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
Revenues	$ 1,990,160	$ 1,843,334	$ 1,726,915	$ 1,629,158	$ 1,721,062	$ 1,690,659	$ 1,615,026	$ 1,528,712	$ 7,189,568	$ 6,555,459	$ 6,187,646
Operating income	145,814	164,972	163,615	149,389	194,361	182,977	175,792	167,890	623,789	721,020	715,694
Net earnings	$ 385,308	$ 100,385	$ 97,869	$ 102,702	$ 117,556	$ 109,425	$ 105,011	$ 100,098	$ 686,263	$ 432,090	$ 427,235
Basic earnings per share (in dollars per share)	$ 3.51	$ 0.92	$ 0.89	$ 0.93	$ 1.05	$ 0.97	$ 0.93	$ 0.89	$ 6.24	$ 3.84	$ 3.69
Diluted earnings per share (in dollars per share)	$ 3.48	$ 0.91	$ 0.88	$ 0.92	$ 1.05	$ 0.97	$ 0.92	$ 0.88	$ 6.18	$ 3.81	$ 3.66