Number Of Banking Offices Acquired

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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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Number of shares of equity interests issued or issuable to acquire entity.

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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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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

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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 intangible assets, excluding goodwill, acquired at the acquisition date.

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The amount of identifiable intangible assets recognized as of the acquisition date.

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For transactions that are recognized separately from the acquisition of assets and assumptions of liabilities in the business combination, such as related to pre-existing relationships with the acquiree, this element represents the disclosure of the amount of acquisition related costs.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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

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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	Feb. 01, 2022 USD ($) office $ / shares shares	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 1,100.0		$ 1,100.0		$ 621.6	$ 621.6
Estimated useful lives of related deposits				12 years
Acquisition related expenses		4.0	$ 6.6	$ 115.0	$ 6.6
Goodwill, Period Increase (Decrease)		0.2
Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years			10 years
Great Western Bank
Business Acquisition [Line Items]
Number of banking offices acquired \| office	174
Total consideration exchanged	$ 1,723.3
Goodwill	478.4
Core deposit intangibles	50.1
Customer relationship intangible	22.8
Great Western Bank \| First Interstate Foundation
Business Acquisition [Line Items]
Acquisition related costs	21.5
Great Western Bank \| Scott Family
Business Acquisition [Line Items]
Acquisition related costs	$ 8.2
Great Western Bank \| Class A Common Stock
Business Acquisition [Line Items]
Number of shares issued in connection with Cascade merger (in shares) \| shares	46,879,601
Price per share of First Interstate Class A common stock (in dollars per share) \| $ / shares	$ 36.76
Great Western Bank \| Customer Relationships
Business Acquisition [Line Items]
Estimated useful lives of related deposits	12 years
Acquisition related costs as defined by ASC 805
Business Acquisition [Line Items]
Acquisition related expenses		$ 0.0		$ 26.8