Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]
Beginning balance at Dec. 31, 2011	$ 1,254,836	$ 59,955	$ 1,179,135	$ 78,599	$ (62,441)	$ (412)
Beginning balance, shares at Dec. 31, 2011		178,683
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	143,627			143,627
Other comprehensive income (loss), net of tax	11,532				11,532
Cash dividends declared on common stock	(128,661)			(128,661)
Effect of stock incentive plan, net	4,604	136	5,596	(65)		(1,063)
Effect of stock incentive plan, net, shares		1,339
Common stock dividend declared	(108,982)		(108,982)
Common stock dividend paid	108,982	3,288	105,694
Common stock issued	216,439	6,115	209,408	(5)		921
Common stock issued, shares		18,416
Ending balance at Dec. 31, 2012	1,502,377	69,494	1,390,851	93,495	(50,909)	(554)
Ending balance, shares at Dec. 31, 2012		198,438
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	131,961			131,961
Other comprehensive income (loss), net of tax	12,657				12,657
Cash dividends declared on common stock	(119,081)			(119,081)
Effect of stock incentive plan, net	5,472	227	6,500	(34)		(1,221)
Effect of stock incentive plan, net, shares		380
Common stock issued	7,654	220	6,024	(1)		1,411
Common stock issued, shares		775
Ending balance at Dec. 31, 2013	1,541,040	69,941	1,403,375	106,340	(38,252)	(364)
Ending balance, shares at Dec. 31, 2013		199,593
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	116,172			116,172
Other comprehensive income (loss), net of tax	(4,243)				(4,243)
Cash dividends declared on common stock	(91,581)			(91,581)
Effect of stock incentive plan, net	5,806	234	6,269	(83)		(614)
Effect of stock incentive plan, net, shares		1,299
Common stock issued	295,823	10,897	284,108	(3)		821
Common stock issued, shares		31,219
Ending balance at Dec. 31, 2014	$ 1,863,017	$ 81,072	$ 1,693,752	$ 130,845	$ (42,495)	$ (157)
Ending balance, shares at Dec. 31, 2014		232,111