Fair Value (Quantitative Information about Level 3 fair value measurements) (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other real estate owned	$ 11,908	$ 10,777
Premises, furniture and equipment held for sale	4,422	1,977
Securities available for sale	2,274,215	2,216,753
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets	0	0
Other real estate owned	11,908	10,777
Securities available for sale	0	0
Level 3 \| Commercial servicing rights
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commercial servicing rights		291
Level 3 \| Premises, furniture and equipment held for sale
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Premises, furniture and equipment held for sale	4,422	1,977
Level 3 \| Interest rate lock commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets	1,539	1,738
Level 3 \| Collateralized Debt Obligations [Member] \| Commercial \| Commercial
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale	6,772	3,212
Level 3 \| Collateralized Debt Obligations [Member] \| Commercial \| Commercial real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale	420	9,480
Level 3 \| Collateralized Debt Obligations [Member] \| Agricultural and agricultural real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale	11,821	8,406
Level 3 \| Collateralized Debt Obligations [Member] \| Residential real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale	590	1,137
Level 3 \| Collateralized Debt Obligations [Member] \| Consumer
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale	$ 1,942	$ 1,234
Weighted Average \| Level 3 \| Interest rate lock commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Closing ratio		0.89
Minimum \| Commercial servicing rights
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	13.28%	13.04%
Maximum \| Commercial servicing rights
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	16.55%	15.49%
Closing Ratio \| Minimum \| Level 3 \| Interest rate lock commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Closing ratio	0.00	0.00
Closing Ratio \| Maximum \| Level 3 \| Interest rate lock commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Closing ratio	0.99	0.99
Closing Ratio \| Heartland Legacy Mortgage Operations \| Weighted Average \| Level 3 \| Interest rate lock commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Closing ratio	0.86
Closing Ratio \| PrimeWest Mortgage Corporation \| Weighted Average \| Level 3 \| Interest rate lock commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Closing ratio	0.95
Appraisal discount \| Minimum \| Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Appraisal discount \| Minimum \| Level 3 \| Commercial \| Commercial
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Appraisal discount \| Minimum \| Level 3 \| Commercial \| Commercial real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Appraisal discount \| Minimum \| Level 3 \| Agricultural and agricultural real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Appraisal discount \| Minimum \| Level 3 \| Residential real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Appraisal discount \| Minimum \| Level 3 \| Premises, furniture and equipment held for sale
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Appraisal discount \| Maximum \| Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	10.00%
Appraisal discount \| Maximum \| Level 3 \| Commercial \| Commercial
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	13.00%	15.00%
Appraisal discount \| Maximum \| Level 3 \| Commercial \| Commercial real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	12.00%	12.00%
Appraisal discount \| Maximum \| Level 3 \| Agricultural and agricultural real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	15.00%	10.00%
Appraisal discount \| Maximum \| Level 3 \| Residential real estate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	24.00%	12.00%
Appraisal discount \| Maximum \| Level 3 \| Premises, furniture and equipment held for sale
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	10.00%	10.00%
Valuation discount \| Minimum \| Level 3 \| Consumer
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	0.00%	0.00%
Valuation discount \| Maximum \| Level 3 \| Consumer
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Pretax discount rate (percent)	14.00%	12.00%

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

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Value of input used to measure derivative asset.

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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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No definition available.

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- Definition

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.

+ References

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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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Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

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