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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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 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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Amount of loss from the write-down of an asset representing the 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) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Goodwill and Intangibles - Narrative (Details) - USD ($)		3 Months Ended
	May 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill		$ 445,600,000	$ 212,800,000	$ 444,700,000	$ 212,800,000
Addition to goodwill from acquisition		0	0
Measurement period adjustment to previously recorded goodwill		900,000	$ 0
Impairment of goodwill		$ 0
Core Deposits
Goodwill [Line Items]
Weighted average useful lives of related deposits		10 years
Core Deposits \| Bank of the Cascades
Goodwill [Line Items]
Weighted average useful lives of related deposits	10 years