Loans (Impaired loans) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Unpaid Contractual Balance
Total impaired loans with a related allowance	$ 28,570	$ 28,570	$ 25,978
Total impaired loans without a related allowance	59,064	59,064	72,511
Total impaired loans held to maturity	87,634	87,634	98,489
Loan Balance
Total impaired loans with a related allowance	26,714	26,714	25,966
Total impaired loans without a related allowance	57,335	57,335	71,407
Total impaired loans held to maturity	84,049	84,049	97,373
Related Allowance Recorded	7,043	7,043	6,753
Average Loan Balance
Total impaired loans with a related allowance	22,842	22,615	21,126
Total impaired loans without a related allowance	61,085	64,480	78,788
Total impaired loans held to maturity	83,927	87,095	99,914
Interest Income Recognized
Total impaired loans with a related allowance	10	50	129
Total impaired loans without a related allowance	332	939	2,325
Total impaired loans held to maturity	342	989	2,454
Commercial
Unpaid Contractual Balance
Total impaired loans with a related allowance	14,462	14,462	17,073
Total impaired loans without a related allowance	24,019	24,019	32,800
Total impaired loans held to maturity	38,481	38,481	49,873
Loan Balance
Total impaired loans with a related allowance	12,606	12,606	17,061
Total impaired loans without a related allowance	22,294	22,294	31,868
Total impaired loans held to maturity	34,900	34,900	48,929
Related Allowance Recorded	3,030	3,030	3,989
Average Loan Balance
Total impaired loans with a related allowance	15,522	15,935	13,761
Total impaired loans without a related allowance	20,964	23,767	44,961
Total impaired loans held to maturity	36,486	39,702	58,722
Interest Income Recognized
Total impaired loans with a related allowance	0	8	23
Total impaired loans without a related allowance	201	648	1,898
Total impaired loans held to maturity	201	656	1,921
Commercial | Commercial
Unpaid Contractual Balance
Total impaired loans with a related allowance	3,190	3,190	2,852
Total impaired loans without a related allowance	4,887	4,887	925
Total impaired loans held to maturity	8,077	8,077	3,777
Loan Balance
Total impaired loans with a related allowance	3,190	3,190	2,840
Total impaired loans without a related allowance	3,767	3,767	872
Total impaired loans held to maturity	6,957	6,957	3,712
Related Allowance Recorded	2,166	2,166	1,318
Average Loan Balance
Total impaired loans with a related allowance	4,885	3,829	3,136
Total impaired loans without a related allowance	2,727	2,017	5,329
Total impaired loans held to maturity	7,612	5,846	8,465
Interest Income Recognized
Total impaired loans with a related allowance	0	1	2
Total impaired loans without a related allowance	0	112	251
Total impaired loans held to maturity	0	113	253
Commercial | Commercial real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	11,272	11,272	14,221
Total impaired loans without a related allowance	19,132	19,132	31,875
Total impaired loans held to maturity	30,404	30,404	46,096
Loan Balance
Total impaired loans with a related allowance	9,416	9,416	14,221
Total impaired loans without a related allowance	18,527	18,527	30,996
Total impaired loans held to maturity	27,943	27,943	45,217
Related Allowance Recorded	864	864	2,671
Average Loan Balance
Total impaired loans with a related allowance	10,637	12,106	10,625
Total impaired loans without a related allowance	18,237	21,750	39,632
Total impaired loans held to maturity	28,874	33,856	50,257
Interest Income Recognized
Total impaired loans with a related allowance	0	7	21
Total impaired loans without a related allowance	201	536	1,647
Total impaired loans held to maturity	201	543	1,668
Agricultural and agricultural real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	10,289	10,289	2,771
Total impaired loans without a related allowance	2,503	2,503	13,959
Total impaired loans held to maturity	12,792	12,792	16,730
Loan Balance
Total impaired loans with a related allowance	10,289	10,289	2,771
Total impaired loans without a related allowance	2,503	2,503	13,959
Total impaired loans held to maturity	12,792	12,792	16,730
Related Allowance Recorded	2,353	2,353	816
Average Loan Balance
Total impaired loans with a related allowance	3,532	2,140	912
Total impaired loans without a related allowance	8,343	10,858	12,722
Total impaired loans held to maturity	11,875	12,998	13,634
Interest Income Recognized
Total impaired loans with a related allowance	0	0	21
Total impaired loans without a related allowance	0	0	157
Total impaired loans held to maturity	0	0	178
Residential real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	1,640	1,640	3,490
Total impaired loans without a related allowance	28,197	28,197	22,408
Total impaired loans held to maturity	29,837	29,837	25,898
Loan Balance
Total impaired loans with a related allowance	1,640	1,640	3,490
Total impaired loans without a related allowance	28,193	28,193	22,236
Total impaired loans held to maturity	29,833	29,833	25,726
Related Allowance Recorded	393	393	497
Average Loan Balance
Total impaired loans with a related allowance	1,633	2,197	3,371
Total impaired loans without a related allowance	27,556	26,006	18,446
Total impaired loans held to maturity	29,189	28,203	21,817
Interest Income Recognized
Total impaired loans with a related allowance	0	10	43
Total impaired loans without a related allowance	112	230	202
Total impaired loans held to maturity	112	240	245
Consumer
Unpaid Contractual Balance
Total impaired loans with a related allowance	2,179	2,179	2,644
Total impaired loans without a related allowance	4,345	4,345	3,344
Total impaired loans held to maturity	6,524	6,524	5,988
Loan Balance
Total impaired loans with a related allowance	2,179	2,179	2,644
Total impaired loans without a related allowance	4,345	4,345	3,344
Total impaired loans held to maturity	6,524	6,524	5,988
Related Allowance Recorded	1,267	1,267	1,451
Average Loan Balance
Total impaired loans with a related allowance	2,155	2,343	3,082
Total impaired loans without a related allowance	4,222	3,849	2,659
Total impaired loans held to maturity	6,377	6,192	5,741
Interest Income Recognized
Total impaired loans with a related allowance	10	32	42
Total impaired loans without a related allowance	19	61	68
Total impaired loans held to maturity	$ 29	$ 93	$ 110