Fair Value (Quantitative Information about Level 3 fair value measurements) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities available for sale	$ 2,027,665	$ 2,216,753
Premises, furniture and equipment held for sale	1,477	1,977
Other real estate owned	$ 11,801	$ 10,777
Commercial servicing rights \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	13.09%	13.04%
Commercial servicing rights \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	16.71%	15.49%
Level 3
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities available for sale	$ 0	$ 0
Derivative assets	0	0
Other real estate owned	$ 11,801	$ 10,777
Level 3 \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	10.00%
Level 3 \| Commercial \| Commercial
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	$ 2,923	$ 3,212
Level 3 \| Commercial \| Commercial \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Commercial \| Commercial \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	15.00%
Level 3 \| Commercial \| Commercial real estate
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	$ 8,316	$ 9,480
Level 3 \| Commercial \| Commercial real estate \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Commercial \| Commercial real estate \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	12.00%
Level 3 \| Agricultural and agricultural real estate
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	$ 8,041	$ 8,406
Level 3 \| Agricultural and agricultural real estate \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Agricultural and agricultural real estate \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	10.00%
Level 3 \| Residential real estate
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	$ 1,421	$ 1,137
Level 3 \| Residential real estate \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Residential real estate \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	12.00%	12.00%
Level 3 \| Consumer
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Collateral dependent impaired loans	$ 1,721	$ 1,234
Level 3 \| Consumer \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Consumer \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	14.00%	12.00%
Level 3 \| Commercial servicing rights
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commercial servicing rights	$ 272	$ 291
Level 3 \| Premises, furniture and equipment held for sale
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Premises, furniture and equipment held for sale	$ 1,477	$ 1,977
Level 3 \| Premises, furniture and equipment held for sale \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Level 3 \| Premises, furniture and equipment held for sale \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	10.00%
Level 3 \| Interest rate lock commitments
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative assets	$ 1,959	$ 1,738
Level 3 \| Interest rate lock commitments \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Closing ratio	0.00%	0.00%
Level 3 \| Interest rate lock commitments \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Closing ratio	99.00%	99.00%
Level 3 \| Interest rate lock commitments \| Weighted Average
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Closing ratio	89.00%	89.00%

Collateral Dependent Impaired Loans

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Fair Value Inputs, Closing Rate

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- Definition

Amount of assets held-for-sale that are not part of a disposal group.

+ References

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

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Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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