Loans (Impaired Loans Not Covered by Loss Share Agreements) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unpaid Principal Balance
Impaired loans with a related allowance	$ 19,958	$ 25,978
Impaired loans without a related allowance	64,121	72,511
Total impaired loans held to maturity	84,079	98,489
Loan Balance
Impaired loans with a related allowance	18,101	25,966
Impaired loans without a related allowance	60,367	71,407
Total impaired loans held to maturity	78,468	97,373
Impaired loans, related allowance recorded	4,755	6,753
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	22,949	21,126
Impaired loans without a related allowance	62,296	78,788
Total impaired loans held to maturity	85,245	99,914
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	129	129
Impaired loans without a related allowance	1,029	2,325
Total loans held to maturity	1,158	2,454
Commercial
Unpaid Principal Balance
Impaired loans with a related allowance	14,217	17,073
Impaired loans without a related allowance	20,486	32,800
Total impaired loans held to maturity	34,703	49,873
Loan Balance
Impaired loans with a related allowance	12,360	17,061
Impaired loans without a related allowance	18,760	31,868
Total impaired loans held to maturity	31,120	48,929
Impaired loans, related allowance recorded	2,379	3,989
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	15,086	13,761
Impaired loans without a related allowance	22,734	44,961
Total impaired loans held to maturity	37,820	58,722
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	73	23
Impaired loans without a related allowance	679	1,898
Total loans held to maturity	752	1,921
Commercial | Commercial
Unpaid Principal Balance
Impaired loans with a related allowance	2,292	2,852
Impaired loans without a related allowance	6,243	925
Total impaired loans held to maturity	8,535	3,777
Loan Balance
Impaired loans with a related allowance	2,292	2,840
Impaired loans without a related allowance	5,123	872
Total impaired loans held to maturity	7,415	3,712
Impaired loans, related allowance recorded	1,613	1,318
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	3,607	3,136
Impaired loans without a related allowance	2,586	5,329
Total impaired loans held to maturity	6,193	8,465
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	39	2
Impaired loans without a related allowance	165	251
Total loans held to maturity	204	253
Commercial | Commercial real estate
Unpaid Principal Balance
Impaired loans with a related allowance	11,925	14,221
Impaired loans without a related allowance	14,243	31,875
Total impaired loans held to maturity	26,168	46,096
Loan Balance
Impaired loans with a related allowance	10,068	14,221
Impaired loans without a related allowance	13,637	30,996
Total impaired loans held to maturity	23,705	45,217
Impaired loans, related allowance recorded	766	2,671
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	11,479	10,625
Impaired loans without a related allowance	20,148	39,632
Total impaired loans held to maturity	31,627	50,257
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	34	21
Impaired loans without a related allowance	514	1,647
Total loans held to maturity	548	1,668
Agricultural and agricultural real estate
Unpaid Principal Balance
Impaired loans with a related allowance	1,539	2,771
Impaired loans without a related allowance	13,793	13,959
Total impaired loans held to maturity	15,332	16,730
Loan Balance
Impaired loans with a related allowance	1,539	2,771
Impaired loans without a related allowance	11,765	13,959
Total impaired loans held to maturity	13,304	16,730
Impaired loans, related allowance recorded	546	816
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	3,437	912
Impaired loans without a related allowance	9,654	12,722
Total impaired loans held to maturity	13,091	13,634
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	0	21
Impaired loans without a related allowance	0	157
Total loans held to maturity	0	178
Residential real estate
Unpaid Principal Balance
Impaired loans with a related allowance	1,568	3,490
Impaired loans without a related allowance	25,573	22,408
Total impaired loans held to maturity	27,141	25,898
Loan Balance
Impaired loans with a related allowance	1,568	3,490
Impaired loans without a related allowance	25,573	22,236
Total impaired loans held to maturity	27,141	25,726
Impaired loans, related allowance recorded	430	497
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	2,056	3,371
Impaired loans without a related allowance	26,024	18,446
Total impaired loans held to maturity	28,080	21,817
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	15	43
Impaired loans without a related allowance	277	202
Total loans held to maturity	292	245
Consumer
Unpaid Principal Balance
Impaired loans with a related allowance	2,634	2,644
Impaired loans without a related allowance	4,269	3,344
Total impaired loans held to maturity	6,903	5,988
Loan Balance
Impaired loans with a related allowance	2,634	2,644
Impaired loans without a related allowance	4,269	3,344
Total impaired loans held to maturity	6,903	5,988
Impaired loans, related allowance recorded	1,400	1,451
Year-to-Date Avg. Loan Balance
Impaired loans with a related allowance	2,370	3,082
Impaired loans without a related allowance	3,884	2,659
Total impaired loans held to maturity	6,254	5,741
Year-to-Date Interest Income Recognized
Impaired loans with a related allowance	41	42
Impaired loans without a related allowance	73	68
Total loans held to maturity	$ 114	$ 110