Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Certain Loans and Debt Securities Acquired in Transfer, Allowance for Credit Losses Due to Subsequent Impairment					$ 501	$ 287
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 75,336	$ 81,371	$ 74,200	$ 85,339
Provision charged to operating expense	1,340	(2,001)	2,435	(7,001)
Less loans charged-off	(1,508)	(2,626)	(3,368)	(5,417)
Add back recoveries of loans previously charged-off	1,384	1,522	3,285	5,345
Ending balance	76,552	78,266	76,552	78,266
Allowance for loan losses:
Loans individually evaluated for impairment					8,721	5,792
Loans collectively evaluated for impairment					67,831	68,408
Ending balance	75,336	81,371	74,200	85,339	76,552	74,200
Total loans:
Individually evaluated for impairment					100,493	78,480
Individually evaluated for impairment					5,003,453	4,818,963
Total loans					5,103,946	4,897,443
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	53,659	59,830	53,884	63,923
Provision charged to operating expense	461	(2,011)	(570)	(5,386)
Less loans charged-off	(610)	(1,158)	(795)	(2,243)
Add back recoveries of loans previously charged-off	425	651	1,416	1,018
Ending balance	53,935	57,312	53,935	57,312
Allowance for loan losses:
Loans individually evaluated for impairment					4,824	3,961
Loans collectively evaluated for impairment					49,111	49,923
Ending balance	53,659	59,830	53,884	63,923	53,935	53,884
Total loans:
Individually evaluated for impairment					75,435	62,775
Individually evaluated for impairment					3,264,732	3,203,306
Total loans					3,340,167	3,266,081
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,499	5,377	5,035	6,193
Provision charged to operating expense	646	346	1,771	(232)
Less loans charged-off	(837)	(934)	(2,138)	(1,780)
Add back recoveries of loans previously charged-off	520	558	1,160	1,166
Ending balance	5,828	5,347	5,828	5,347
Allowance for loan losses:
Loans individually evaluated for impairment					0	0
Loans collectively evaluated for impairment					5,828	5,035
Ending balance	5,499	5,377	5,035	6,193	5,828	5,035
Total loans:
Individually evaluated for impairment					0	0
Individually evaluated for impairment					799,126	762,471
Total loans					799,126	762,471
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	15,196	15,701	14,307	14,747
Provision charged to operating expense	224	(494)	1,217	(1,566)
Less loans charged-off	(61)	(534)	(435)	(1,330)
Add back recoveries of loans previously charged-off	438	313	708	3,135
Ending balance	15,797	14,986	15,797	14,986
Allowance for loan losses:
Loans individually evaluated for impairment					3,279	1,190
Loans collectively evaluated for impairment					12,518	13,117
Ending balance	15,196	15,701	14,307	14,747	15,797	14,307
Total loans:
Individually evaluated for impairment					23,573	14,526
Individually evaluated for impairment					795,546	725,547
Total loans					819,119	740,073
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	982	463	974	476
Provision charged to operating expense	9	158	17	183
Less loans charged-off	0	0	0	(64)
Add back recoveries of loans previously charged-off	1	0	1	26
Ending balance	992	621	992	621
Allowance for loan losses:
Loans individually evaluated for impairment					618	641
Loans collectively evaluated for impairment					374	333
Ending balance	982	463	974	476	992	974
Total loans:
Individually evaluated for impairment					1,485	1,179
Individually evaluated for impairment					141,144	123,680
Total loans					142,629	124,859
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0	0
Provision charged to operating expense	0	0	0	0
Less loans charged-off	0	0	0	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
Loans collectively evaluated for impairment					0	0
Ending balance	$ 0	$ 0	$ 0	$ 0	0	0
Total loans:
Individually evaluated for impairment					0	0
Individually evaluated for impairment					2,905	3,959
Total loans					$ 2,905	$ 3,959