Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions			3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 08, 2019 USD ($) segment office $ / shares shares	Aug. 16, 2018 USD ($) office $ / shares shares	Dec. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Change in goodwill			$ 0.1
Goodwill			621.6		$ 621.6	$ 546.7	$ 444.7
Number of operating segments | segment					1
Acquisition related expenses				$ 12.4	$ 20.3	12.4	$ 27.2
Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Net	$ 3.0		$ 62.1		62.1	56.9
Finite-lived intangible assets acquired					16.6	$ 15.7
Estimated useful lives of related deposits	10 years
Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Core deposit intangible assets	$ 13.6
Number of banking offices | office	11
Total consideration paid	$ 157.3
Goodwill	$ 73.0
Idaho Independent Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares	0.50
Common stock issued (in shares) | shares	3,871,422
Business Acquisition, Share Price | $ / shares	$ 40.64
Idaho Independent Bank [Member] | Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Change in goodwill					$ 0.3
Core deposit intangible assets		$ 15.7
Number of banking offices | office		20
Cash paid as consideration		$ 3.0
Total consideration paid		176.3
Goodwill		$ 100.7
Number of operating segments | segment	1
Inland Northwest Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares		0.516
Common stock issued (in shares) | shares		3,840,000
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
Community 1st Bank [Member]
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Liabilities	$ 110.1
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Assets	$ 129.1
Number of banking offices | office	3
Total consideration paid	$ 18.8
Goodwill	2.3
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Assets	16.5
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets	78.8
Business Combination, Acquired Receivables, Estimated Uncollectible	0.7
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Liabilities	$ 112.6
Community 1st Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares	0.3784
Common stock issued (in shares) | shares	463,134
Business Acquisition, Share Price | $ / shares	$ 40.64