Note 12 - Operating Results by Quarter (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues	$ 1,621,015	$ 1,586,494	$ 1,539,957	$ 1,440,180	$ 1,609,511	$ 1,601,156	$ 1,547,867	$ 1,406,908	$ 6,187,646	$ 6,165,441	$ 5,584,571
Operating income	192,893	193,846	173,735	155,220	182,905	172,100	159,230	117,307	715,694	631,542	576,708
Net earnings	$ 116,746	$ 115,139	$ 103,419	$ 91,932	$ 110,306	$ 102,414	$ 93,408	$ 68,664	$ 427,235	$ 374,792	$ 342,382
Basic earnings per share (in dollars per share)	$ 1.02	$ 1	$ 0.89	$ 0.79	$ 0.94	$ 0.87	$ 0.80	$ 0.59	$ 3.69	$ 3.20	$ 2.92
Diluted earnings per share (in dollars per share)	$ 1.01	$ 0.99	$ 0.88	$ 0.78	$ 0.93	$ 0.87	$ 0.79	$ 0.58	$ 3.66	$ 3.16	$ 2.87