Loans (Impaired loans) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Unpaid Contractual Balance
Total impaired loans with a related allowance	$ 13,456	$ 13,456	$ 19,958
Total impaired loans without a related allowance	78,191	78,191	64,121
Total impaired loans held to maturity	91,647	91,647	84,079
Loan Balance
Total impaired loans with a related allowance	13,456	13,456	18,101
Total impaired loans without a related allowance	74,213	74,213	60,367
Total impaired loans held to maturity	87,669	87,669	78,468
Related Allowance Recorded	6,003	6,003	4,755
Avg. Loan Balance
Total impaired loans with a related allowance	13,214	15,944	22,949
Total impaired loans without a related allowance	69,101	66,590	62,296
Total impaired loans held to maturity	82,315	82,534	85,245
Interest Income Recognized
Total impaired loans with a related allowance	35	113	129
Total impaired loans without a related allowance	406	880	1,029
Total impaired loans held to maturity	441	993	1,158
Commercial
Unpaid Contractual Balance
Total impaired loans with a related allowance	7,516	7,516	14,217
Total impaired loans without a related allowance	31,768	31,768	20,486
Total impaired loans held to maturity	39,284	39,284	34,703
Loan Balance
Total impaired loans with a related allowance	7,516	7,516	12,360
Total impaired loans without a related allowance	30,313	30,313	18,760
Total impaired loans held to maturity	37,829	37,829	31,120
Related Allowance Recorded	4,379	4,379	2,379
Avg. Loan Balance
Total impaired loans with a related allowance	6,062	9,228	15,086
Total impaired loans without a related allowance	24,963	22,154	22,734
Total impaired loans held to maturity	31,025	31,382	37,820
Interest Income Recognized
Total impaired loans with a related allowance	20	72	73
Total impaired loans without a related allowance	262	478	679
Total impaired loans held to maturity	282	550	752
Commercial | Commercial
Unpaid Contractual Balance
Total impaired loans with a related allowance	6,768	6,768	2,292
Total impaired loans without a related allowance	18,003	18,003	6,243
Total impaired loans held to maturity	24,771	24,771	8,535
Loan Balance
Total impaired loans with a related allowance	6,768	6,768	2,292
Total impaired loans without a related allowance	16,646	16,646	5,123
Total impaired loans held to maturity	23,414	23,414	7,415
Related Allowance Recorded	4,165	4,165	1,613
Avg. Loan Balance
Total impaired loans with a related allowance	5,173	3,593	3,607
Total impaired loans without a related allowance	13,113	9,307	2,586
Total impaired loans held to maturity	18,286	12,900	6,193
Interest Income Recognized
Total impaired loans with a related allowance	16	52	39
Total impaired loans without a related allowance	182	277	165
Total impaired loans held to maturity	198	329	204
Commercial | Commercial real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	748	748	11,925
Total impaired loans without a related allowance	13,765	13,765	14,243
Total impaired loans held to maturity	14,513	14,513	26,168
Loan Balance
Total impaired loans with a related allowance	748	748	10,068
Total impaired loans without a related allowance	13,667	13,667	13,637
Total impaired loans held to maturity	14,415	14,415	23,705
Related Allowance Recorded	214	214	766
Avg. Loan Balance
Total impaired loans with a related allowance	889	5,635	11,479
Total impaired loans without a related allowance	11,850	12,847	20,148
Total impaired loans held to maturity	12,739	18,482	31,627
Interest Income Recognized
Total impaired loans with a related allowance	4	20	34
Total impaired loans without a related allowance	80	201	514
Total impaired loans held to maturity	84	221	548
Agricultural and agricultural real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	2,043	2,043	1,539
Total impaired loans without a related allowance	18,002	18,002	13,793
Total impaired loans held to maturity	20,045	20,045	15,332
Loan Balance
Total impaired loans with a related allowance	2,043	2,043	1,539
Total impaired loans without a related allowance	15,479	15,479	11,765
Total impaired loans held to maturity	17,522	17,522	13,304
Related Allowance Recorded	260	260	546
Avg. Loan Balance
Total impaired loans with a related allowance	3,000	2,369	3,437
Total impaired loans without a related allowance	15,837	14,726	9,654
Total impaired loans held to maturity	18,837	17,095	13,091
Interest Income Recognized
Total impaired loans with a related allowance	2	2	0
Total impaired loans without a related allowance	1	4	0
Total impaired loans held to maturity	3	6	0
Residential real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	869	869	1,568
Total impaired loans without a related allowance	22,778	22,778	25,573
Total impaired loans held to maturity	23,647	23,647	27,141
Loan Balance
Total impaired loans with a related allowance	869	869	1,568
Total impaired loans without a related allowance	22,778	22,778	25,573
Total impaired loans held to maturity	23,647	23,647	27,141
Related Allowance Recorded	279	279	430
Avg. Loan Balance
Total impaired loans with a related allowance	1,006	1,278	2,056
Total impaired loans without a related allowance	22,779	24,780	26,024
Total impaired loans held to maturity	23,785	26,058	28,080
Interest Income Recognized
Total impaired loans with a related allowance	4	10	15
Total impaired loans without a related allowance	121	320	277
Total impaired loans held to maturity	125	330	292
Consumer
Unpaid Contractual Balance
Total impaired loans with a related allowance	3,028	3,028	2,634
Total impaired loans without a related allowance	5,643	5,643	4,269
Total impaired loans held to maturity	8,671	8,671	6,903
Loan Balance
Total impaired loans with a related allowance	3,028	3,028	2,634
Total impaired loans without a related allowance	5,643	5,643	4,269
Total impaired loans held to maturity	8,671	8,671	6,903
Related Allowance Recorded	1,085	1,085	1,400
Avg. Loan Balance
Total impaired loans with a related allowance	3,146	3,069	2,370
Total impaired loans without a related allowance	5,522	4,930	3,884
Total impaired loans held to maturity	8,668	7,999	6,254
Interest Income Recognized
Total impaired loans with a related allowance	9	29	41
Total impaired loans without a related allowance	22	78	73
Total impaired loans held to maturity	$ 31	$ 107	$ 114