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Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details) | 12 Months Ended
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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 |

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