Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				$ 382	$ 287
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 74,200	$ 85,339	$ 100,511
Provision charged to operating expense	6,822	(6,622)	(6,125)
Less loans charged-off	(11,637)	(13,995)	(20,513)
Add back recoveries of loans previously charged-off	7,432	9,478	11,466
Ending balance	76,817	74,200	85,339
Allowance for loan losses:
Loans individually evaluated for impairment				11,575	5,792	$ 8,929
Loans collectively evaluated for impairment				65,242	68,408	76,410
Ending balance	74,200	85,339	100,511	76,817	74,200	85,339
Total loans:
Individually evaluated for impairment				91,965	78,480	110,572
Individually evaluated for impairment				5,101,356	4,778,135	4,193,420
Total Loans				5,193,321	4,856,615	4,303,992
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	53,884	63,923	75,782
Provision charged to operating expense	(645)	(10,348)	(7,722)
Less loans charged-off	(4,076)	(3,014)	(10,224)
Add back recoveries of loans previously charged-off	3,133	3,323	6,087
Ending balance	52,296	53,884	63,923
Allowance for loan losses:
Loans individually evaluated for impairment				4,794	3,961	7,339
Loans collectively evaluated for impairment				47,502	49,923	56,584
Ending balance	53,884	63,923	75,782	52,296	53,884	63,923
Total loans:
Individually evaluated for impairment				66,516	62,775	96,862
Individually evaluated for impairment				3,346,546	3,162,478	2,745,393
Total Loans				3,413,062	3,225,253	2,842,255
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,035	6,193	7,141
Provision charged to operating expense	3,243	1,382	1,605
Less loans charged-off	(5,683)	(4,887)	(4,612)
Add back recoveries of loans previously charged-off	2,549	2,347	2,059
Ending balance	5,144	5,035	6,193
Allowance for loan losses:
Loans individually evaluated for impairment				0	0	0
Loans collectively evaluated for impairment				5,144	5,035	6,193
Ending balance	5,035	6,193	7,141	5,144	5,035	6,193
Total loans:
Individually evaluated for impairment				0	0	0
Individually evaluated for impairment				844,353	762,471	671,587
Total Loans				844,353	762,471	671,587
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	14,307	14,747	17,085
Provision charged to operating expense	4,376	1,809	41
Less loans charged-off	(1,657)	(6,030)	(5,672)
Add back recoveries of loans previously charged-off	1,749	3,781	3,293
Ending balance	18,775	14,307	14,747
Allowance for loan losses:
Loans individually evaluated for impairment				6,487	1,190	1,504
Loans collectively evaluated for impairment				12,288	13,117	13,243
Ending balance	14,307	14,747	17,085	18,775	14,307	14,747
Total loans:
Individually evaluated for impairment				24,471	14,526	13,585
Individually evaluated for impairment				767,945	725,547	662,959
Total Loans				792,416	740,073	676,544
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	974	476	503
Provision charged to operating expense	(152)	535	(49)
Less loans charged-off	(221)	(64)	(5)
Add back recoveries of loans previously charged-off	1	27	27
Ending balance	602	974	476
Allowance for loan losses:
Loans individually evaluated for impairment				294	641	86
Loans collectively evaluated for impairment				308	333	390
Ending balance	974	476	503	602	974	476
Total loans:
Individually evaluated for impairment				978	1,179	125
Individually evaluated for impairment				141,173	123,680	111,747
Total Loans				142,151	124,859	111,872
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0
Provision charged to operating expense	0	0	0
Less loans charged-off	0	0	0
Add back recoveries of loans previously charged-off	0	0	0
Ending balance	0	0	0
Allowance for loan losses:
Loans individually evaluated for impairment				0	0	0
Loans collectively evaluated for impairment				0	0	0
Ending balance	$ 0	$ 0	$ 0	0	0	0
Total loans:
Individually evaluated for impairment				0	0	0
Individually evaluated for impairment				1,339	3,959	1,734
Total Loans				$ 1,339	$ 3,959	$ 1,734