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Shares transferred at acquisition-date in consideration of business combination.

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Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, OREO

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Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

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The amount of property, plant, and equipment recognized as of the acquisition date.

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Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Number of shares of stock issued during the period pursuant to acquisitions.

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Acquisitions - Narrative (Details) - USD ($)				3 Months Ended	6 Months Ended
Acquisitions - Narrative (Details) - USD ($)	Jun. 01, 2018	Feb. 01, 2018	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Business Acquisition [Line Items]
Value of stock transaction					$ 137,000	$ 0
Bank Mutual
Business Acquisition [Line Items]
Number of shares received by shareholders (in shares)		0.422
Number of shares issued			19,500,000
Value of stock transaction		$ 482,000,000
Goodwill acquired		$ 173,000,000			173,000,000
Amortization period		10 years
Additions to finite-lived intangible assets				$ 58,000,000
BKMU Assets Acquired Liabilities Assumed IncreaseDecrease [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		$ 6,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, OREO		$ 1,000,000
Business Combination, Acquired Receivables, IncreaseDecrease		less than $1 million
Anderson Insurance Investment Agency [Domain]
Business Acquisition [Line Items]
Goodwill acquired	$ 7,000,000			7,000,000
Amortization period	7 years
Value of acquisition	$ 10,000,000
Additions to finite-lived intangible assets	$ 3,000,000			3,000,000
Diversified Insurance Solutions
Business Acquisition [Line Items]
Goodwill acquired					$ 10,000,000
Amortization period		10 years
Value of acquisition				$ 19,000,000
Additions to finite-lived intangible assets		$ 8,000,000