Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 115,902	$ 124,446	$ 112,581	$ 120,480
Provision charged to operating expense	12,000	15,400	23,250	30,400
Less loans charged-off	(26,745)	(16,102)	(35,832)	(28,441)
Add back recoveries of loans previously charged-off	1,637	835	2,795	2,140
Ending balance	102,794	124,579	102,794	124,579
Allowance for loan losses:
Loans individually evaluated for impairment	17,943	43,892	17,943	43,892
Loans collectively evaluated for impairment	84,851	80,687	84,851	80,687
Ending balance	102,794	124,579	102,794	124,579
Total loans:
Individually evaluated for impairment	162,500	242,017	162,500	242,017
Individually evaluated for impairment	4,007,463	4,039,243	4,007,463	4,039,243
Total loans	4,169,963	4,281,260	4,169,963	4,281,260	4,186,549
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	89,128	92,350	87,396	84,181
Provision charged to operating expense	10,419	10,941	16,801	23,096
Less loans charged-off	(22,350)	(13,157)	(27,506)	(17,388)
Add back recoveries of loans previously charged-off	526	109	1,032	354
Ending balance	77,723	90,243	77,723	90,243
Allowance for loan losses:
Loans individually evaluated for impairment	15,199	29,903	15,199	29,903
Loans collectively evaluated for impairment	62,524	60,340	62,524	60,340
Ending balance	77,723	90,243	77,723	90,243
Total loans:
Individually evaluated for impairment	150,026	208,485	150,026	208,485
Individually evaluated for impairment	2,538,281	2,585,238	2,538,281	2,585,238
Total loans	2,688,307	2,793,723	2,688,307	2,793,723
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,212	8,992	8,594	9,332
Provision charged to operating expense	784	563	193	1,251
Less loans charged-off	(1,210)	(1,499)	(2,522)	(2,959)
Add back recoveries of loans previously charged-off	488	470	1,009	902
Ending balance	7,274	8,526	7,274	8,526
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	7,274	8,526	7,274	8,526
Ending balance	7,274	8,526	7,274	8,526
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	621,212	626,184	621,212	626,184
Total loans	621,212	626,184	621,212	626,184
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	18,380	21,790	15,325	25,354
Provision charged to operating expense	866	3,798	6,307	6,255
Less loans charged-off	(3,180)	(1,407)	(5,692)	(8,049)
Add back recoveries of loans previously charged-off	609	253	735	874
Ending balance	16,675	24,434	16,675	24,434
Allowance for loan losses:
Loans individually evaluated for impairment	2,339	13,740	2,339	13,740
Loans collectively evaluated for impairment	14,336	10,694	14,336	10,694
Ending balance	16,675	24,434	16,675	24,434
Total loans:
Individually evaluated for impairment	11,974	32,609	11,974	32,609
Individually evaluated for impairment	708,036	691,549	708,036	691,549
Total loans	720,010	724,158	720,010	724,158
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,182	1,314	1,266	1,613
Provision charged to operating expense	(69)	98	(51)	(202)
Less loans charged-off	(5)	(39)	(112)	(45)
Add back recoveries of loans previously charged-off	14	3	19	10
Ending balance	1,122	1,376	1,122	1,376
Allowance for loan losses:
Loans individually evaluated for impairment	405	249	405	249
Loans collectively evaluated for impairment	717	1,127	717	1,127
Ending balance	1,122	1,376	1,122	1,376
Total loans:
Individually evaluated for impairment	500	923	500	923
Individually evaluated for impairment	137,615	132,975	137,615	132,975
Total loans	138,115	133,898	138,115	133,898
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0	0
Provision charged to operating expense	0	0	0	0
Less loans charged-off	0	0	0	0
Add back recoveries of loans previously charged-off	0	0	0	0
Ending balance	0	0	0	0
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	0	0	0	0
Ending balance	0	0	0	0
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	2,319	3,297	2,319	3,297
Total loans	$ 2,319	$ 3,297	$ 2,319	$ 3,297