Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 85,339	$ 100,511
Provision charged to operating expense	(5,000)	500
Less loans charged-off	(2,791)	(6,020)
Add back recoveries of loans previously charged-off	3,823	2,913
Ending balance	81,371	97,904
Allowance for loan losses:
Loans individually evaluated for impairment	7,494	12,117
Loans collectively evaluated for impairment	73,877	85,787
Ending balance	81,371	97,904
Total loans:
Individually evaluated for impairment	101,929	130,674
Individually evaluated for impairment	4,262,909	4,093,885
Total loans	4,364,838	4,224,559	4,344,853
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	63,923	75,782
Provision charged to operating expense	(3,375)	(1,035)
Less loans charged-off	(1,085)	(4,143)
Add back recoveries of loans previously charged-off	367	1,062
Ending balance	59,830	71,666
Allowance for loan losses:
Loans individually evaluated for impairment	5,952	8,367
Loans collectively evaluated for impairment	53,878	63,299
Ending balance	59,830	71,666
Total loans:
Individually evaluated for impairment	86,772	114,690
Individually evaluated for impairment	2,788,421	2,671,943
Total loans	2,875,193	2,786,633
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,193	7,140
Provision charged to operating expense	(578)	537
Less loans charged-off	(846)	(1,062)
Add back recoveries of loans previously charged-off	608	473
Ending balance	5,377	7,088
Allowance for loan losses:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	5,377	7,088
Ending balance	5,377	7,088
Total loans:
Individually evaluated for impairment	0	0
Individually evaluated for impairment	670,406	636,364
Total loans	670,406	636,364
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	14,747	17,085
Provision charged to operating expense	(1,072)	1,021
Less loans charged-off	(796)	(811)
Add back recoveries of loans previously charged-off	2,822	1,375
Ending balance	15,701	18,670
Allowance for loan losses:
Loans individually evaluated for impairment	1,458	3,725
Loans collectively evaluated for impairment	14,243	14,945
Ending balance	15,701	18,670
Total loans:
Individually evaluated for impairment	14,464	15,832
Individually evaluated for impairment	692,773	673,012
Total loans	707,237	688,844
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	476	504
Provision charged to operating expense	25	(23)
Less loans charged-off	(64)	(4)
Add back recoveries of loans previously charged-off	26	3
Ending balance	463	480
Allowance for loan losses:
Loans individually evaluated for impairment	84	25
Loans collectively evaluated for impairment	379	455
Ending balance	463	480
Total loans:
Individually evaluated for impairment	693	152
Individually evaluated for impairment	107,683	111,259
Total loans	108,376	111,411
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0
Provision charged to operating expense	0	0
Less loans charged-off	0	0
Add back recoveries of loans previously charged-off	0	0
Ending balance	0	0
Allowance for loan losses:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	0	0
Ending balance	0	0
Total loans:
Individually evaluated for impairment	0	0
Individually evaluated for impairment	3,626	1,307
Total loans	$ 3,626	$ 1,307