Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) note	Dec. 31, 2017 USD ($) note	Dec. 31, 2016 USD ($) note
Loans and Leases Receivable, Troubled Debt Restructuring
Loans and Leases Receivable, Impaired, Commitment to Lend	$ 0	$ 0
Loans renegotiated in troubled debt restructurings	23,400,000	44,500,000
Loans renegotiated in troubled debt restructurings, non-accrual loans	17,800,000	31,900,000
Loans renegotiated in troubled debt restructurings, accrual loans	$ 5,600,000	$ 12,600,000
Number of notes under troubled debt restructurings | note	5	24	35
Concessions in troubled debt restructurings, amount	$ 4,100,000	$ 12,900,000	$ 20,100,000
Charge-offs directly related to modified loans	$ 0	$ 0	$ 0
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	3	5	18
Concessions in troubled debt restructurings, amount	$ 3,600,000	$ 2,800,000	$ 7,900,000
Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note			1
Concessions in troubled debt restructurings, amount			$ 100,000
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	1	1
Concessions in troubled debt restructurings, amount	$ 200,000	$ 800,000
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	1
Concessions in troubled debt restructurings, amount	$ 300,000
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		17	13
Concessions in troubled debt restructurings, amount		$ 9,200,000	$ 8,100,000
Balance of notes for which there was a payment default	1,300,000
Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		1
Concessions in troubled debt restructurings, amount		$ 100,000
Construction Loans [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note			1
Concessions in troubled debt restructurings, amount			$ 3,700,000
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note			2
Concessions in troubled debt restructurings, amount			$ 300,000
Interest Only Period Concession
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	3,600,000	2,700,000	4,800,000
Interest Only Period Concession | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	3,600,000	1,500,000	400,000
Interest Only Period Concession | Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest Only Period Concession | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0
Interest Only Period Concession | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0
Interest Only Period Concession | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		1,200,000	4,400,000
Interest Only Period Concession | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Interest Only Period Concession | Construction Loans [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest Only Period Concession | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	3,300,000	10,000,000
Extension of terms or maturity | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	400,000	5,500,000
Extension of terms or maturity | Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			100,000
Extension of terms or maturity | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	800,000
Extension of terms or maturity | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0
Extension of terms or maturity | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		2,000,000	400,000
Extension of terms or maturity | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		100,000
Extension of terms or maturity | Construction Loans [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			3,700,000
Extension of terms or maturity | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			300,000
Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0	200,000
Interest rate adjustment | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0	200,000
Interest rate adjustment | Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest rate adjustment | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0
Interest rate adjustment | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0
Interest rate adjustment | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0	0
Interest rate adjustment | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Interest rate adjustment | Construction Loans [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest rate adjustment | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Other
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	500,000	6,900,000	5,100,000
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	900,000	1,800,000
Other | Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Other | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	200,000	0
Other | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	$ 300,000
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		6,000,000	3,300,000
Other | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		$ 0
Other | Construction Loans [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Other | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			$ 0