Consolidated Statements of Shareholders' Equity (USD $) In Thousands, except Share data	Common Stock [Member]	Capital Surplus [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2010	$ 208,704	$ 224,976	$ (14,453)	$ 803,017	$ 1,222,244
Balance (in shares) at Dec. 31, 2010	83,481,737
Net income				37,569	37,569
Change in fair value of available-for-sale securities, net of tax effect			26,989		26,989
Change in pension funding status, net of tax effect			(14,797)		(14,797)
Comprehensive income					49,761
Exercise of stock options	5	15			20
Exercise of stock options (in shares)	2,217				2,225
Income tax benefit (expense) from exercise of stock options		(50)			(50)
Recognition of stock compensation	13	2,670			2,683
Recognition of stock compensation (in shares)	5,000
Repurchase of stock	(13)	(44)			(57)
Repurchase of stock (in shares)	(5,158)
Cash dividends declared				(11,689)	(11,689)
Balance at Dec. 31, 2011	208,709	227,567	(2,261)	828,897	1,262,912
Balance (in shares) at Dec. 31, 2011	83,483,796
Net income				84,295	84,295
Change in fair value of available-for-sale securities, net of tax effect			1,599		1,599
Change in pension funding status, net of tax effect			(7,984)		(7,984)
Comprehensive income					77,910
Issuance of stock	27,381	81,296			108,677
Issuance of stock (in shares)	10,952,381
Exercise of stock options	17	67			84
Exercise of stock options (in shares)	6,533
Income tax benefit (expense) from exercise of stock options		(32)			(32)
Recognition of stock compensation	281	3,073			3,354
Recognition of stock compensation (in shares)	112,315
Repurchase of stock	(13)	(62)			(75)
Repurchase of stock (in shares)	(5,158)
Cash dividends declared				(3,778)	(3,778)
Balance at Dec. 31, 2012	236,375	311,909	(8,646)	909,414	1,449,052
Balance (in shares) at Dec. 31, 2012	94,549,867
Net income				94,115	94,115
Change in fair value of available-for-sale securities, net of tax effect			(38,065)		(38,065)
Change in pension funding status, net of tax effect			16,752		16,752
Comprehensive income					72,802
Exercise of stock options	147	765			912
Exercise of stock options (in shares)	58,982
Income tax benefit (expense) from exercise of stock options		139			139
Recognition of stock compensation	1,557	87			1,644
Recognition of stock compensation (in shares)	622,842
Cash dividends declared				(11,419)	(11,419)
Balance at Dec. 31, 2013	$ 238,079	$ 312,900	$ (29,959)	$ 992,110	$ 1,513,130
Balance (in shares) at Dec. 31, 2013	95,231,691