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Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions |  |  |  | 12 Months Ended |  |
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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.

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