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Goodwill and Intangibles - Narrative (Details) - USD ($)		3 Months Ended		6 Months Ended
	May 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill		$ 445,600,000	$ 444,300,000	$ 445,600,000	$ 444,700,000	$ 212,800,000
Addition to provisional goodwill from acquisition		0	231,500,000
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