Loans (Impaired loans) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Unpaid Contractual Balance
Total impaired loans with a related allowance	$ 20,084	$ 19,958
Total impaired loans without a related allowance	69,290	64,121
Total impaired loans held to maturity	89,374	84,079
Loan Balance
Total impaired loans with a related allowance	18,228	18,101
Total impaired loans without a related allowance	65,344	60,367
Total impaired loans held to maturity	83,572	78,468
Related Allowance Recorded	5,471	4,755
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	18,206	22,949
Total impaired loans without a related allowance	63,072	62,296
Total impaired loans held to maturity	81,278	85,245
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	20	129
Total impaired loans without a related allowance	278	1,029
Total impaired loans held to maturity	298	1,158
Commercial
Unpaid Contractual Balance
Total impaired loans with a related allowance	13,996	14,217
Total impaired loans without a related allowance	21,510	20,486
Total impaired loans held to maturity	35,506	34,703
Loan Balance
Total impaired loans with a related allowance	12,140	12,360
Total impaired loans without a related allowance	19,785	18,760
Total impaired loans held to maturity	31,925	31,120
Related Allowance Recorded	3,161	2,379
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	11,992	15,086
Total impaired loans without a related allowance	19,328	22,734
Total impaired loans held to maturity	31,320	37,820
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	8	73
Total impaired loans without a related allowance	146	679
Total impaired loans held to maturity	154	752
Commercial | Commercial
Unpaid Contractual Balance
Total impaired loans with a related allowance	2,816	2,292
Total impaired loans without a related allowance	7,308	6,243
Total impaired loans held to maturity	10,124	8,535
Loan Balance
Total impaired loans with a related allowance	2,816	2,292
Total impaired loans without a related allowance	6,189	5,123
Total impaired loans held to maturity	9,005	7,415
Related Allowance Recorded	2,425	1,613
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	2,472	3,607
Total impaired loans without a related allowance	5,449	2,586
Total impaired loans held to maturity	7,921	6,193
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	0	39
Total impaired loans without a related allowance	49	165
Total impaired loans held to maturity	49	204
Commercial | Commercial real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	11,180	11,925
Total impaired loans without a related allowance	14,202	14,243
Total impaired loans held to maturity	25,382	26,168
Loan Balance
Total impaired loans with a related allowance	9,324	10,068
Total impaired loans without a related allowance	13,596	13,637
Total impaired loans held to maturity	22,920	23,705
Related Allowance Recorded	736	766
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	9,520	11,479
Total impaired loans without a related allowance	13,879	20,148
Total impaired loans held to maturity	23,399	31,627
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	8	34
Total impaired loans without a related allowance	97	514
Total impaired loans held to maturity	105	548
Agricultural and agricultural real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	1,536	1,539
Total impaired loans without a related allowance	17,388	13,793
Total impaired loans held to maturity	18,924	15,332
Loan Balance
Total impaired loans with a related allowance	1,536	1,539
Total impaired loans without a related allowance	15,360	11,765
Total impaired loans held to maturity	16,896	13,304
Related Allowance Recorded	787	546
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	1,537	3,437
Total impaired loans without a related allowance	12,954	9,654
Total impaired loans held to maturity	14,491	13,091
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	0	0
Total impaired loans without a related allowance	1	0
Total impaired loans held to maturity	1	0
Residential real estate
Unpaid Contractual Balance
Total impaired loans with a related allowance	1,693	1,568
Total impaired loans without a related allowance	26,635	25,573
Total impaired loans held to maturity	28,328	27,141
Loan Balance
Total impaired loans with a related allowance	1,693	1,568
Total impaired loans without a related allowance	26,631	25,573
Total impaired loans held to maturity	28,324	27,141
Related Allowance Recorded	386	430
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	1,608	2,056
Total impaired loans without a related allowance	26,878	26,024
Total impaired loans held to maturity	28,486	28,080
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	3	15
Total impaired loans without a related allowance	109	277
Total impaired loans held to maturity	112	292
Consumer
Unpaid Contractual Balance
Total impaired loans with a related allowance	2,859	2,634
Total impaired loans without a related allowance	3,757	4,269
Total impaired loans held to maturity	6,616	6,903
Loan Balance
Total impaired loans with a related allowance	2,859	2,634
Total impaired loans without a related allowance	3,568	4,269
Total impaired loans held to maturity	6,427	6,903
Related Allowance Recorded	1,137	1,400
Year- to- Date Avg. Loan Balance
Total impaired loans with a related allowance	3,069	2,370
Total impaired loans without a related allowance	3,912	3,884
Total impaired loans held to maturity	6,981	6,254
Year- to- Date Interest Income Recognized
Total impaired loans with a related allowance	9	41
Total impaired loans without a related allowance	22	73
Total impaired loans held to maturity	$ 31	$ 114