Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Apr. 08, 2019 USD ($) segment office $ / shares shares	Aug. 16, 2018 USD ($) office $ / shares shares	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 621.6	$ 546.7	$ 444.7
Number of operating segments | segment			1
Idaho Independent Bank, Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	$ 0.0
Investment securities	0.5
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	(9.8)
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale	0.0
Allowance for loan losses	6.3
Premises and equipment	4.8
Other real estate owned (“OREO”)	2.0
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned	0.0
Core deposit intangible assets	13.6
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset	(2.6)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense	2.6
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets	(0.7)
Total assets acquired	14.1
Deposits	0.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities	0.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements	0.0
Total liabilities assumed	2.8
Net assets acquired	$ 11.3
Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Number of banking offices | office	11
Cash and cash equivalents	$ 270.7
Investment securities	63.2
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	337.8
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale	0.5
Allowance for loan losses	0.0
Premises and equipment	21.3
Other real estate owned (“OREO”)	2.4
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned	15.2
Core deposit intangible assets	13.6
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset	0.6
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense	17.8
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets	7.9
Total assets acquired	733.2
Deposits	596.6
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities	4.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements	30.4
Total liabilities assumed	648.9
Net assets acquired	84.3
Class A common stock	157.3
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, As Recorded By Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale	$ 0.5
Allowance for loan losses	(6.3)
Premises and equipment	16.5
Other real estate owned (“OREO”)	0.4
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned	15.2
Core deposit intangible assets	0.0
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset	3.2
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense	15.2
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets	8.6
Total assets acquired	719.1
Deposits	596.5
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities	4.0
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements	30.4
Total liabilities assumed	646.1
Net assets acquired	$ 73.0
Community 1st Bank [Member]
Business Acquisition [Line Items]
Number of banking offices | office	3
Total consideration paid	$ 18.8
Goodwill	$ 2.3
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
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	$ 270.7	$ 31.2
Investment securities	62.7	3.1
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	$ 347.6	727.9
Allowance for loan losses		(8.0)
Premises and equipment		14.5
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets		2.4
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets		29.3
Total assets acquired		800.7
Deposits		696.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities		8.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		13.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities		5.2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		(1.2)
Total liabilities assumed		721.2
Net assets acquired		79.5
Northwest Bank Acquisition, Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		0.0
Investment securities		0.0
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment		(14.8)
Allowance for loan losses		8.0
Premises and equipment		0.0
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets		13.3
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets		(10.0)
Total assets acquired		(3.2)
Deposits		0.2
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities		(0.4)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		0.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities		(0.8)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		1.6
Total liabilities assumed		0.7
Net assets acquired		$ (3.9)
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Number of banking offices | office		20
Cash and cash equivalents		$ 31.2
Investment securities		3.1
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment		713.1
Allowance for loan losses		0.0
Premises and equipment		14.5
Other real estate owned (“OREO”)		0.6
Core deposit intangible assets		15.7
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets		19.3
Total assets acquired		797.5
Deposits		696.3
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities		7.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		13.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities		4.4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		0.4
Total liabilities assumed		721.9
Net assets acquired		75.6
Cash		3.0
Class A common stock		173.3
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
Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years
Core Deposits Intangibles | Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years
Core Deposits Intangibles | Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Estimated useful lives of related deposits		10 years