Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) note	Dec. 31, 2016 USD ($) note	Dec. 31, 2015 USD ($) note
Loans and Leases Receivable, Troubled Debt Restructuring
Loans renegotiated in troubled debt restructurings	$ 44,500,000	$ 49,600,000
Loans renegotiated in troubled debt restructurings, non-accrual loans	31,900,000	27,300,000
Loans renegotiated in troubled debt restructurings, accrual loans	$ 12,600,000	$ 22,300,000
Number of notes under troubled debt restructurings | note	24	35	15
Concessions in troubled debt restructurings, amount	$ 12,900,000	$ 20,100,000	$ 13,200,000
Charge-offs directly related to modified loans	$ 0	$ 0	$ 0
Number of notes for which there was a payment default | note	1
Balance of notes for which there was a payment default	$ 1,300,000
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	5	18	2
Concessions in troubled debt restructurings, amount	$ 2,800,000	$ 7,900,000	$ 700,000
Commercial Construction
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
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
Concessions in troubled debt restructurings, amount	$ 800,000
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	6	20
Concessions in troubled debt restructurings, amount	$ 3,600,000	$ 11,700,000
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			$ 0
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	17	13	12
Concessions in troubled debt restructurings, amount	$ 9,200,000	$ 8,100,000	$ 12,500,000
Number of notes for which there was a payment default | note	1
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	2
Concessions in troubled debt restructurings, amount	$ 100,000	$ 300,000
Interest Only Period Concession
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	2,700,000	4,800,000	0
Interest Only Period Concession | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	1,500,000	400,000	0
Interest Only Period Concession | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
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
Interest Only Period Concession | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	1,500,000	400,000
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	0
Interest Only Period Concession | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0
Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	3,300,000	10,000,000	9,300,000
Extension of terms or maturity | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	400,000	5,500,000	600,000
Extension of terms or maturity | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		3,700,000
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	800,000
Extension of terms or maturity | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	1,200,000	9,300,000
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	8,700,000
Extension of terms or maturity | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	100,000	300,000
Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	200,000	3,300,000
Interest rate adjustment | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	200,000	0
Interest rate adjustment | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
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
Interest rate adjustment | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	200,000
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	3,300,000
Interest rate adjustment | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0
Other
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	6,900,000	5,100,000	600,000
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	900,000	1,800,000	100,000
Other | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
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	0
Other | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	900,000	1,800,000
Other | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	6,000,000	3,300,000	$ 500,000
Other | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	$ 0	$ 0