Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 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					$ 344	$ 287
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 76,552	$ 78,266	$ 74,200	$ 85,339
Provision charged to operating expense	1,098	261	3,533	(6,740)
Less loans charged-off	(4,547)	(6,274)	(7,916)	(11,691)
Add back recoveries of loans previously charged-off	1,153	1,978	4,439	7,323
Ending balance	74,256	74,231	74,256	74,231
Allowance for loan losses:
Loans individually evaluated for impairment					8,917	5,792
Loans collectively evaluated for impairment					65,339	68,408
Ending balance	76,552	78,266	74,200	85,339	74,256	74,200
Total loans:
Individually evaluated for impairment					92,233	78,480
Individually evaluated for impairment					5,084,247	4,818,963
Total loans					5,176,480	4,897,443
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	53,935	57,312	53,884	63,923
Provision charged to operating expense	(1,149)	(3,839)	(1,718)	(9,225)
Less loans charged-off	(2,219)	(147)	(3,014)	(2,390)
Add back recoveries of loans previously charged-off	389	999	1,804	2,017
Ending balance	50,956	54,325	50,956	54,325
Allowance for loan losses:
Loans individually evaluated for impairment					3,301	3,961
Loans collectively evaluated for impairment					47,655	49,923
Ending balance	53,935	57,312	53,884	63,923	50,956	53,884
Total loans:
Individually evaluated for impairment					67,195	62,775
Individually evaluated for impairment					3,342,109	3,203,306
Total loans					3,409,304	3,266,081
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,828	5,347	5,035	6,193
Provision charged to operating expense	430	423	2,201	191
Less loans charged-off	(1,537)	(1,437)	(3,675)	(3,217)
Add back recoveries of loans previously charged-off	653	495	1,813	1,661
Ending balance	5,374	4,828	5,374	4,828
Allowance for loan losses:
Loans individually evaluated for impairment					0	0
Loans collectively evaluated for impairment					5,374	5,035
Ending balance	5,828	5,347	5,035	6,193	5,374	5,035
Total loans:
Individually evaluated for impairment					0	0
Individually evaluated for impairment					831,961	762,471
Total loans					831,961	762,471
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	15,797	14,986	14,307	14,747
Provision charged to operating expense	2,004	3,598	3,220	2,032
Less loans charged-off	(791)	(4,678)	(1,227)	(6,008)
Add back recoveries of loans previously charged-off	111	484	821	3,619
Ending balance	17,121	14,390	17,121	14,390
Allowance for loan losses:
Loans individually evaluated for impairment					5,154	1,190
Loans collectively evaluated for impairment					11,967	13,117
Ending balance	15,797	14,986	14,307	14,747	17,121	14,307
Total loans:
Individually evaluated for impairment					23,565	14,526
Individually evaluated for impairment					755,083	725,547
Total loans					778,648	740,073
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	992	621	974	476
Provision charged to operating expense	(187)	67	(170)	250
Less loans charged-off	0	0	0	(64)
Add back recoveries of loans previously charged-off	0	0	1	26
Ending balance	805	688	805	688
Allowance for loan losses:
Loans individually evaluated for impairment					462	641
Loans collectively evaluated for impairment					343	333
Ending balance	992	621	974	476	805	974
Total loans:
Individually evaluated for impairment					1,473	1,179
Individually evaluated for impairment					153,382	123,680
Total loans					154,855	124,859
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0	0
Provision charged to operating expense	0	12	0	12
Less loans charged-off	0	(12)	0	(12)
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					1,712	3,959
Total loans					$ 1,712	$ 3,959