Income Tax (Details) (USD $) In Thousands, 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
Components of income tax expense (benefit):
Current tax (benefit) expense									$ 21,218	$ 21,688	$ (13,547)
Deferred tax expense (benefit)									(3,656)	(3,783)	15,838
Total	5,342	4,655	4,367	3,198	5,664	7,244	2,670	2,327	17,562	17,905	2,291
Deferred tax assets:
Allowance for loan and lease losses	30,027				20,910				30,027	20,910
Supplemental executive retirement plan	6,967				6,564				6,967	6,564
Stock option and restricted stock	682				989				682	989
OREO costs	3,801				3,209				3,801	3,209
Nonaccrual interest	193				222				193	222
Security impairment	0				1,041				0	1,041
Other	557				632				557	632
Total deferred tax assets	42,227				33,567				42,227	33,567
Deferred tax liabilities:
Asset purchase tax basis difference	(19,408)				(14,812)				(19,408)	(14,812)
FHLB stock dividends	(1,963)				(1,977)				(1,963)	(1,977)
Purchase accounting	(745)				(1,030)				(745)	(1,030)
Deferred loan fees	(1,755)				(1,517)				(1,755)	(1,517)
Unrealized gain on investment securities	(11,150)				(14,291)				(11,150)	(14,291)
Depreciation	(1,870)				(1,517)				(1,870)	(1,517)
Total deferred tax liabilities	36,891				35,144				36,891	35,144
Net deferred tax (liability) asset	5,336				(1,577)				5,336	(1,577)
Reconciliation of effective income tax rate with federal statutory tax rate
Income tax based on statutory rate									22,297	23,080	11,576
Income tax based on statutory rate, percent									35.00%	35.00%	35.00%
Tax credits									(504)	(608)	(808)
Tax credits, percent									(1.00%)	(1.00%)	(2.00%)
Tax exempt instrument									(3,906)	(3,824)	(3,744)
Tax exempt instrument, percent									(6.00%)	(6.00%)	(11.00%)
Life insurance proceeds									(1,001)	(766)	(735)
Life insurance proceeds, percent									(2.00%)	(1.00%)	(2.00%)
Bargain purchase									0	(1,036)	(5,383)
Bargain purchase, percent									0.00%	(2.00%)	(16.00%)
Other, net									676	1,059	1,385
Other, net, percent									1.00%	2.00%	3.00%
Income tax provision (benefit)									17,562	17,905	2,291
Income tax provision (benefit), percent									27.00%	27.00%	7.00%
Unrecognized tax position	0								0
Interest and penalties on unrecognized tax benefits									$ 0