Consolidated Statements of Changes in Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock	Capital Surplus	Retained Earnings	Accumulated Other Comprehensive (Loss)/Gain, net	Total
Balance at Dec. 31, 2008	$ 105,793	$ 101,210	$ 411,579	$ (9,083)	$ 609,499
Balance, shares at Dec. 31, 2008	31,769,679
Comprehensive income
Net income			74,775		74,775
Net change from retirement benefit plans, net of tax				1,473	1,473
Unrealized net holding gain (or loss) on securities, net of reclassifications and tax				10,608	10,608
Comprehensive income					86,856
Cash dividends declared ($0.96 per common share)			(32,011)		(32,011)
Common stock activity, long-term incentive plan, including excess income tax benefit, shares	125,774
Common stock activity, long-term incentive plan, including excess income tax benefit	419	5,528			5,947
Common stock issued, shares	4,945,000
Common stock issued	16,467	150,905			167,372
Balance at Dec. 31, 2009	122,679	257,643	454,343	2,998	837,663
Balance, shares at Dec. 31, 2009	36,840,453
Comprehensive income
Net income			52,206		52,206
Net change from retirement benefit plans, net of tax				(2,463)	(2,463)
Unrealized net holding gain (or loss) on securities, net of reclassifications and tax				(1,154)	(1,154)
Comprehensive income					48,589
Cash dividends declared ($0.96 per common share)			(35,721)		(35,721)
Common stock activity, long-term incentive plan, including excess income tax benefit, shares	52,823
Common stock activity, long-term incentive plan, including excess income tax benefit	176	5,841			6,017
Balance at Dec. 31, 2010	122,855	263,484	470,828	(619)	856,548
Balance, shares at Dec. 31, 2010	36,893,276
Comprehensive income
Net income			76,759		76,759
Net change from retirement benefit plans, net of tax				(59,072)	(59,072)
Unrealized net holding gain (or loss) on securities, net of reclassifications and tax				33,246	33,246
Net unrealized gains on derivatives and hedging activities, net of reclassification				(65)	(65)
Comprehensive income					50,868
Cash dividends declared ($0.96 per common share)			(70,617)		(70,617)
Common stock issued in stock offering, shares	6,958,143
Common stock issued in stock offering	23,170	190,824			213,994
Common stock issued in business combination, shares	40,794,261
Common stock issued in business combination	135,845	1,172,199			1,308,044
Common stock activity, long-term incentive plan, including excess income tax benefit, shares	59,816
Common stock activity, long-term incentive plan, including excess income tax benefit	199	8,127			8,326
Balance at Dec. 31, 2011	$ 282,069	$ 1,634,634	$ 476,970	$ (26,510)	$ 2,367,163
Balance, shares at Dec. 31, 2011	84,705,496