Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 100,511	$ 112,581
Provision charged to operating expense	500	11,250
Less loans charged-off	(6,020)	(9,087)
Add back recoveries of loans previously charged-off	2,913	1,158
Ending balance	97,904	115,902
Allowance for loan losses:
Loans individually evaluated for impairment	12,117
Loans collectively evaluated for impairment	85,787
Ending balance	97,904	115,902
Total loans:
Individually evaluated for impairment	130,674
Individually evaluated for impairment	4,093,885
Total loans	4,224,559	4,158,616	4,223,912
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	75,782	87,396
Provision charged to operating expense	(1,035)	6,382
Less loans charged-off	(4,143)	(5,156)
Add back recoveries of loans previously charged-off	1,062	506
Ending balance	71,666	89,128
Allowance for loan losses:
Loans individually evaluated for impairment	8,367
Loans collectively evaluated for impairment	63,299
Ending balance	71,666	89,128
Total loans:
Individually evaluated for impairment	114,690
Individually evaluated for impairment	2,671,943
Total loans	2,786,633	2,714,979
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,140	8,594
Provision charged to operating expense	537	(591)
Less loans charged-off	(1,062)	(1,312)
Add back recoveries of loans previously charged-off	473	521
Ending balance	7,088	7,212
Allowance for loan losses:
Loans individually evaluated for impairment	0
Loans collectively evaluated for impairment	7,088
Ending balance	7,088	7,212
Total loans:
Individually evaluated for impairment	0
Individually evaluated for impairment	636,364
Total loans	636,364	606,073
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	17,073	15,325
Provision charged to operating expense	1,015	5,441
Less loans charged-off	(811)	(2,512)
Add back recoveries of loans previously charged-off	1,375	126
Ending balance	18,652	18,380
Allowance for loan losses:
Loans individually evaluated for impairment	3,725
Loans collectively evaluated for impairment	14,927
Ending balance	18,652	18,380
Total loans:
Individually evaluated for impairment	15,832
Individually evaluated for impairment	673,012
Total loans	688,844	708,397
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	504	1,266
Provision charged to operating expense	(23)	18
Less loans charged-off	(4)	(107)
Add back recoveries of loans previously charged-off	3	5
Ending balance	480	1,182
Allowance for loan losses:
Loans individually evaluated for impairment	25
Loans collectively evaluated for impairment	455
Ending balance	480	1,182
Total loans:
Individually evaluated for impairment	152
Individually evaluated for impairment	111,259
Total loans	111,411	128,599
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	12	0
Provision charged to operating expense	6	0
Less loans charged-off	0	0
Add back recoveries of loans previously charged-off	0	0
Ending balance	18	0
Allowance for loan losses:
Loans individually evaluated for impairment	0
Loans collectively evaluated for impairment	18
Ending balance	18	0
Total loans:
Individually evaluated for impairment	0
Individually evaluated for impairment	1,307
Total loans	$ 1,307	$ 568

Reflects the gross amount charged against earnings during the period as credit losses (bad debt expense) based on review of the loan and lease portfolio and application of the methodology for estimating credit losses.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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