Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions					12 Months Ended
		Oct. 11, 2018 USD ($)	Aug. 16, 2018 USD ($) $ / shares shares	May 30, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Cash Paid For Cascade Unvested Restricted Stock				$ 2.2
Shares Issued For Cascade Unvested Restricted Stock | shares				168,000
Cash Paid Related To Cascade Options				$ 9.3
Goodwill					$ 546.7	$ 444.7	$ 212.8
Number of operating segments | segment					1
Expense Related To Restricted Awards Excluded From Consideration Paid				2.4
Restricted Stock Expense Included In Consideration Paid				5.5
Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Number of banking offices		11
Estimate of Payments to Acquire Businesses, Gross		$ 181.3
Community 1st Bank [Member]
Business Acquisition [Line Items]
Number of banking offices		3
Estimate of Payments to Acquire Businesses, Gross		$ 21.5
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			$ 31.2
Investment securities			3.1
Business Combinations, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans Held for Investment			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
Cascade Bank Acquisition, As Recorded By Cascade Bank
Business Acquisition [Line Items]
Cash and cash equivalents				246.8
Investment securities				476.7
Loans held for investment				2,111.0
Mortgage loans held for sale				10.3
Allowance for loan losses				(24.0)
Premises and equipment				46.6
Other real estate owned (“OREO”)				1.2
Core deposit intangible assets				0.0
Deferred tax assets, net				47.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets				98.6
Total assets acquired				3,014.8
Deposits				2,669.9
Accounts payable and accrued expense				62.2
Total liabilities assumed				2,732.1
Net assets acquired				282.7
Cascade Bank Acquisition, Fair Value Adjustments
Business Acquisition [Line Items]
Cash and cash equivalents				0.0
Investment securities				4.9
Loans held for investment				(31.7)
Mortgage loans held for sale				0.0
Allowance for loan losses				24.0
Premises and equipment				0.1
Other real estate owned (“OREO”)				0.0
Core deposit intangible assets				48.0
Deferred tax assets, net				(20.9)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets				2.1
Total assets acquired				26.5
Deposits				(0.9)
Accounts payable and accrued expense				1.9
Total liabilities assumed				1.0
Net assets acquired				25.5
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Cash and cash equivalents				246.8
Investment securities				481.6
Loans held for investment				2,079.3
Mortgage loans held for sale				10.3
Allowance for loan losses				0.0
Premises and equipment				46.7
Other real estate owned (“OREO”)				1.2
Core deposit intangible assets				48.0
Deferred tax assets, net				26.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets				100.7
Total assets acquired				3,041.3
Deposits				2,669.0
Accounts payable and accrued expense				64.1
Total liabilities assumed				2,733.1
Net assets acquired				308.2
Cash				155.0
Total consideration paid				541.0
Goodwill				232.8
Cascade Bank Acquisition, as Recorded by the Company | Class A Common Stock
Business Acquisition [Line Items]
Class A common stock				386.0
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	[1]		(14.8)
Allowance for loan losses	[2]		8.0
Premises and equipment	[3]		(0.5)
Other real estate owned (“OREO”)			0.3
Core deposit intangible assets	[4]		13.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets			(10.0)
Total assets acquired			(3.7)
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	[5]		1.5
Total liabilities assumed			0.6
Net assets acquired			(4.3)
Northwest Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
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			14.0
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			797.0
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			0.3
Total liabilities assumed			721.8
Net assets acquired			75.2
Cash			3.0
Class A common stock			$ 173.3
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
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
Cascade Bank | Class A Common Stock
Business Acquisition [Line Items]
Class A common stock				$ 173.3
Common stock issued (in shares) | shares				3,837,540
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]
Business Acquisition [Line Items]
Number of banking offices				46
Core Deposits Intangibles | Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Estimated useful lives of related deposits			10 years
Core Deposits Intangibles | Bank of the Cascades [Member]
Business Acquisition [Line Items]
Estimated useful lives of related deposits				10 years

[1] (1) Write down of the book value of loans to their estimated fair values. The fair value of the loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans. The fair value of collateral dependent loans acquired with deteriorated credit quality was estimated based on the Company’s analysis of the fair value of each loan’s underlying collateral, discounted using market-derived rates of return with consideration given to the period of time and costs associated with foreclosure and disposition of the collateral.

[2] (2) Adjustment to remove the Northwest allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (1) above.

[3] (3) Write down of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon broker’s opinion of value.

[4] (4) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm.

[5] (9) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition.