Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions					1 Months Ended	2 Months Ended	12 Months Ended
	Oct. 11, 2018 USD ($) $ / shares	Aug. 16, 2018 USD ($) $ / shares shares	May 30, 2017 USD ($) $ / shares shares	Aug. 12, 2016 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 05, 2018 $ / shares shares
Business Acquisition [Line Items]
Cash Paid Related To Cascade Options			$ 9.3
Cash Paid For Cascade Unvested Restricted Stock			$ 2.2
Shares Issued For Cascade Unvested Restricted Stock | shares			168,000
Expense Related To Restricted Awards Excluded From Consideration Paid			$ 2.4
Restricted Stock Expense Included In Consideration Paid			$ 5.5
Goodwill							$ 546.7	$ 444.7	$ 212.8
Number of operating segments | segment							1
Acquisition related expenses							$ 12.4	$ 27.2	$ 2.8
Community 1st Bank [Member]
Business Acquisition [Line Items]
Estimate value per share of Acquisition | $ / shares	$ 17.20
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Estimate value per share of Acquisition | $ / shares	$ 22.73
Bank of the Cascades [Member]
Business Acquisition [Line Items]
Number of banking offices			46
Class A Common Stock | Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Conversion Share Rate, Shares | shares										0.5000
Estimate value per share of Acquisition | $ / shares										$ 45.45
Class A Common Stock | Community 1st Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Conversion Share Rate, Shares | shares										0.3784
Estimate value per share of Acquisition | $ / shares										$ 45.45
Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired			$ 15.7	$ 48.0
Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Number of banking offices	11
Estimate of Payments to Acquire Businesses, Gross	$ 181.3
Northwest Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
Core deposit intangible assets		$ 15.7
Cash paid as consideration		$ 3.0
Cascade Bank
Business Acquisition [Line Items]
Revenue reported by Cascade					$ 12.9
Net income reported by Cascade					$ 3.0
Cascade Bank | Class A Common Stock
Business Acquisition [Line Items]
Common stock issued (in shares) | shares			3,837,540
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Core deposit intangible assets			$ 48.0
Cash paid as consideration			155.0
Total consideration paid			541.0
Goodwill			$ 232.8
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Number of banking offices		20
Total consideration paid		$ 176.3
Goodwill		$ 101.1
Number of operating segments | segment							1
Revenue reported by Cascade						$ 5.9
Net income reported by Cascade						$ 2.1
Inland Northwest Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares		0.516
Business Acquisition, Share Price | $ / shares		$ 45.15
Inland Northwest Bank [Member] | Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits		10 years
Bank of the Cascades [Member]
Business Acquisition [Line Items]
Cash received by Cascade shareholders (in dollars per share) | $ / shares			$ 1.91
Common stock issued (in shares) | shares			11,300,000
Bank of the Cascades [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares			0.14864
Business Acquisition, Share Price | $ / shares			$ 34.30
Bank of the Cascades [Member] | Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits			10 years
Community 1st Bank [Member]
Business Acquisition [Line Items]
Number of banking offices	3
Estimate of Payments to Acquire Businesses, Gross	$ 21.5
Employee Benefit Plan Expense [Member]
Business Acquisition [Line Items]
Acquisition related expenses							$ 2.3
Legal and Professional Expenses [Member]
Business Acquisition [Line Items]
Acquisition related expenses							$ 21.8