Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Certain Loans and Debt Securities Acquired in Transfer, Allowance for Credit Losses Due to Subsequent Impairment			$ 393	$ 287
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 76,817	$ 74,200
Provision charged to operating expense	4,000	1,095
Less loans charged-off	(4,302)	(1,861)
Add back recoveries of loans previously charged-off	3,409	1,902
Ending balance	79,924	75,336
Allowance for loan losses:
Loans individually evaluated for impairment			13,947	11,575
Loans collectively evaluated for impairment			65,977	65,242
Ending balance	76,817	74,200	79,924	76,817
Total loans:
Individually evaluated for impairment			88,644	91,965
Individually evaluated for impairment			5,102,825	5,101,356
Total loans			5,191,469	5,193,321
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	52,296	53,884
Provision charged to operating expense	(16,081)	(1,031)
Less loans charged-off	(1,922)	(186)
Add back recoveries of loans previously charged-off	2,359	992
Ending balance	36,652	53,659
Allowance for loan losses:
Loans individually evaluated for impairment			5,582	4,794
Loans collectively evaluated for impairment			31,070	47,502
Ending balance	52,296	53,884	36,652	52,296
Total loans:
Individually evaluated for impairment			63,587	66,516
Individually evaluated for impairment			3,310,551	3,346,546
Total loans			3,374,138	3,413,062
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,144	5,035
Provision charged to operating expense	1,229	1,125
Less loans charged-off	(1,892)	(1,301)
Add back recoveries of loans previously charged-off	775	640
Ending balance	5,256	5,499
Allowance for loan losses:
Loans individually evaluated for impairment			0	0
Loans collectively evaluated for impairment			5,256	5,144
Ending balance	5,144	5,035	5,256	5,144
Total loans:
Individually evaluated for impairment			0	0
Individually evaluated for impairment			865,455	844,353
Total loans			865,455	844,353
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	18,775	14,307
Provision charged to operating expense	17,690	993
Less loans charged-off	(488)	(374)
Add back recoveries of loans previously charged-off	275	270
Ending balance	36,252	15,196
Allowance for loan losses:
Loans individually evaluated for impairment			8,176	6,487
Loans collectively evaluated for impairment			28,076	12,288
Ending balance	18,775	14,307	36,252	18,775
Total loans:
Individually evaluated for impairment			24,263	24,471
Individually evaluated for impairment			800,780	767,945
Total loans			825,043	792,416
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	602	974
Provision charged to operating expense	1,162	8
Less loans charged-off	0	0
Add back recoveries of loans previously charged-off	0	0
Ending balance	1,764	982
Allowance for loan losses:
Loans individually evaluated for impairment			189	294
Loans collectively evaluated for impairment			1,575	308
Ending balance	602	974	1,764	602
Total loans:
Individually evaluated for impairment			794	978
Individually evaluated for impairment			125,496	141,173
Total loans			126,290	142,151
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	0	0
Total loans:
Individually evaluated for impairment			0	0
Individually evaluated for impairment			543	1,339
Total loans			$ 543	$ 1,339