Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 78,266	$ 98,528	$ 85,339	$ 100,511
Provision charged to operating expense	261	(3,000)	(6,740)	(2,125)
Less loans charged-off	(6,274)	(4,915)	(11,691)	(14,830)
Add back recoveries of loans previously charged-off	1,978	2,377	7,323	9,434
Ending balance	74,231	92,990	74,231	92,990
Allowance for loan losses:
Loans individually evaluated for impairment	6,570	13,361	6,570	13,361
Loans collectively evaluated for impairment	67,661	79,629	67,661	79,629
Ending balance	74,231	92,990	74,231	92,990
Total loans:
Individually evaluated for impairment	85,058	111,576	85,058	111,576
Individually evaluated for impairment	4,769,324	4,220,516	4,769,324	4,220,516
Total loans	4,854,382	4,332,092	4,854,382	4,332,092	4,344,853
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	57,312	71,503	63,923	75,782
Provision charged to operating expense	(3,839)	(2,820)	(9,225)	(4,864)
Less loans charged-off	(147)	(2,129)	(2,390)	(8,299)
Add back recoveries of loans previously charged-off	999	1,399	2,017	5,334
Ending balance	54,325	67,953	54,325	67,953
Allowance for loan losses:
Loans individually evaluated for impairment	4,805	9,216	4,805	9,216
Loans collectively evaluated for impairment	49,520	58,737	49,520	58,737
Ending balance	54,325	67,953	54,325	67,953
Total loans:
Individually evaluated for impairment	70,555	97,717	70,555	97,717
Individually evaluated for impairment	3,162,534	2,755,298	3,162,534	2,755,298
Total loans	3,233,089	2,853,015	3,233,089	2,853,015
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,347	7,040	6,193	7,140
Provision charged to operating expense	423	289	191	1,405
Less loans charged-off	(1,437)	(1,083)	(3,217)	(3,444)
Add back recoveries of loans previously charged-off	495	484	1,661	1,629
Ending balance	4,828	6,730	4,828	6,730
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0	0
Loans collectively evaluated for impairment	4,828	6,730	4,828	6,730
Ending balance	4,828	6,730	4,828	6,730
Total loans:
Individually evaluated for impairment	0	0	0	0
Individually evaluated for impairment	745,482	672,184	745,482	672,184
Total loans	745,482	672,184	745,482	672,184
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	14,986	19,475	14,747	17,085
Provision charged to operating expense	3,598	(390)	2,032	1,409
Less loans charged-off	(4,678)	(1,703)	(6,008)	(3,083)
Add back recoveries of loans previously charged-off	484	474	3,619	2,445
Ending balance	14,390	17,856	14,390	17,856
Allowance for loan losses:
Loans individually evaluated for impairment	1,445	4,126	1,445	4,126
Loans collectively evaluated for impairment	12,945	13,730	12,945	13,730
Ending balance	14,390	17,856	14,390	17,856
Total loans:
Individually evaluated for impairment	13,669	13,743	13,669	13,743
Individually evaluated for impairment	723,239	667,673	723,239	667,673
Total loans	736,908	681,416	736,908	681,416
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	621	510	476	504
Provision charged to operating expense	67	(79)	250	(75)
Less loans charged-off	0	0	(64)	(4)
Add back recoveries of loans previously charged-off	0	20	26	26
Ending balance	688	451	688	451
Allowance for loan losses:
Loans individually evaluated for impairment	320	19	320	19
Loans collectively evaluated for impairment	368	432	368	432
Ending balance	688	451	688	451
Total loans:
Individually evaluated for impairment	834	116	834	116
Individually evaluated for impairment	135,753	123,449	135,753	123,449
Total loans	136,587	123,565	136,587	123,565
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0	0
Provision charged to operating expense	12	0	12	0
Less loans charged-off	(12)	0	(12)	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	2,316	1,912	2,316	1,912
Total loans	$ 2,316	$ 1,912	$ 2,316	$ 1,912