Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 98,528	$ 102,794	$ 100,511	$ 112,581
Provision charged to operating expense	(3,000)	9,500	(2,125)	32,750
Less loans charged-off	(4,915)	(14,813)	(14,830)	(50,645)
Add back recoveries of loans previously charged-off	2,377	1,525	9,434	4,320
Ending balance	92,990	99,006	92,990	99,006
Allowance for loan losses:
Loans individually evaluated for impairment	13,361	14,684	13,361	14,684
Loans collectively evaluated for impairment	79,629	84,322	79,629	84,322
Ending balance	92,990	99,006	92,990	99,006
Total loans:
Individually evaluated for impairment	111,576	154,458	111,576	154,458
Individually evaluated for impairment	4,220,516	4,025,593	4,220,516	4,025,593
Total loans	4,332,092	4,180,051	4,332,092	4,180,051	4,223,912
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	71,503	77,723	75,782	87,396
Provision charged to operating expense	(2,820)	5,371	(4,864)	22,172
Less loans charged-off	(2,129)	(9,138)	(8,299)	(36,645)
Add back recoveries of loans previously charged-off	1,399	387	5,334	1,420
Ending balance	67,953	74,343	67,953	74,343
Allowance for loan losses:
Loans individually evaluated for impairment	9,216	11,348	9,216	11,348
Loans collectively evaluated for impairment	58,737	62,995	58,737	62,995
Ending balance	67,953	74,343	67,953	74,343
Total loans:
Individually evaluated for impairment	97,717	140,460	97,717	140,460
Individually evaluated for impairment	2,755,298	2,600,908	2,755,298	2,600,908
Total loans	2,853,015	2,741,368	2,853,015	2,741,368
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,040	7,274	7,140	8,594
Provision charged to operating expense	289	192	1,405	385
Less loans charged-off	(1,083)	(1,340)	(3,444)	(3,862)
Add back recoveries of loans previously charged-off	484	465	1,629	1,474
Ending balance	6,730	6,591	6,730	6,591
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	6,730	6,591	6,730	6,591
Ending balance	6,730	6,591	6,730	6,591
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	672,184	629,757	672,184	629,757
Total loans	672,184	629,757	672,184	629,757
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	19,442	16,675	17,073	15,325
Provision charged to operating expense	(377)	4,222	1,401	10,530
Less loans charged-off	(1,703)	(4,335)	(3,083)	(10,028)
Add back recoveries of loans previously charged-off	474	666	2,445	1,401
Ending balance	17,836	17,228	17,836	17,228
Allowance for loan losses:
Loans individually evaluated for impairment	4,126	2,924	4,126	2,924
Loans collectively evaluated for impairment	13,710	14,304	13,710	14,304
Ending balance	17,836	17,228	17,836	17,228
Total loans:
Individually evaluated for impairment	13,743	13,408	13,743	13,408
Individually evaluated for impairment	667,673	658,692	667,673	658,692
Total loans	681,416	672,100	681,416	672,100
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	510	1,122	504	1,266
Provision charged to operating expense	(79)	(285)	(75)	(337)
Less loans charged-off	0	0	(4)	(110)
Add back recoveries of loans previously charged-off	20	7	26	25
Ending balance	451	844	451	844
Allowance for loan losses:
Loans individually evaluated for impairment	19	412	19	412
Loans collectively evaluated for impairment	432	432	432	432
Ending balance	451	844	451	844
Total loans:
Individually evaluated for impairment	116	590	116	590
Individually evaluated for impairment	123,449	134,877	123,449	134,877
Total loans	123,565	135,467	123,565	135,467
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	33	0	12	0
Provision charged to operating expense	(13)	0	8	0
Less loans charged-off	0	0	0	0
Add back recoveries of loans previously charged-off	0	0	0	0
Ending balance	20	0	20	0
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	20	0	20	0
Ending balance	20	0	20	0
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	1,912	1,359	1,912	1,359
Total loans	$ 1,912	$ 1,359	$ 1,912	$ 1,359