Note 11 - Quarterly Financial Information (Unaudited) (Detail) - Operating Results By Quarter (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
2012:
Operating revenues	$ 1,338,136	$ 1,295,792	$ 1,255,130	$ 1,165,922	$ 1,204,937	$ 1,171,270	$ 1,149,855	$ 1,000,780	$ 5,054,980	$ 4,526,842	$ 3,793,485
Operating income	143,335	133,052	137,215	116,597	122,287	118,680	113,443	89,823	530,200	444,233	347,625
Net earnings	$ 83,977	$ 78,241	$ 80,451	$ 67,684	$ 72,565	$ 68,650	$ 65,696	$ 50,095	$ 310,354	$ 257,006	$ 199,617
Basic earnings per share (in Dollars per share)	$ 0.71	$ 0.66	$ 0.69	$ 0.58	$ 0.62	$ 0.58	$ 0.55	$ 0.41	$ 2.64	$ 2.16	$ 1.60
Diluted earnings per share (in Dollars per share)	$ 0.70	$ 0.65	$ 0.67	$ 0.57	$ 0.61	$ 0.57	$ 0.53	$ 0.40	$ 2.59	$ 2.11	$ 1.56