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Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Aug. 16, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Business Acquisition [Line Items] |  |  |  |  |  |
Business Combination, Acquisition Related Costs |  |  | $ 2.3 | $ 2.3 | $ 2.3 |
Goodwill |  |  | 546.3 | 445.6 | $ 546.7 | $ 444.7
Net increase to goodwill |  |  | (0.4) | $ 0.9 |  |
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Cash and cash equivalents |  | $ 31.2 |  |  |  |
Investment securities |  | 3.1 |  |  |  |
Loans held for investment |  | 727.9 |  |  |  |
Allowance for loan loss |  | (8.0) |  |  |  |
Premises and equipment |  | 14.5 |  |  |  |
Other real estate owned (“OREO”) |  | 0.3 |  |  |  |
Core deposit intangible assets |  | 2.4 |  |  |  |
Other assets |  | 29.3 |  |  |  |
Total assets acquired |  | 800.7 |  |  |  |
Deposits |  | 696.1 |  |  |  |
Accounts Payable and Accrued Expense |  | 8.1 |  |  |  |
Long term debt |  | 13.0 |  |  |  |
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 |  |  |  |
Loans held for investment | [1] | (14.8) |  |  |  |
Allowance for loan loss | [2] | 8.0 |  |  |  |
Premises and equipment |  | 0.0 |  |  |  |
Other real estate owned (“OREO”) |  | 0.3 |  |  |  |
Core deposit intangible assets | [3] | 13.3 |  |  |  |
Other assets | [4] | (10.0) |  |  |  |
Total assets acquired |  | (3.2) |  |  |  |
Deposits | [5] | 0.2 |  |  |  |
Accounts Payable and Accrued Expense | [6] | (0.4) |  |  |  |
Long term debt |  | 0.1 |  |  |  |
Trust preferred securities |  | (0.8) |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability | [7] | 1.6 |  |  |  |
Total liabilities assumed |  | 0.7 |  |  |  |
Net assets acquired |  | (3.9) |  |  |  |
Northwest Bank Acquisition, as Recorded by the Company [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Cash and cash equivalents |  | 31.2 |  |  |  |
Investment securities |  | 3.1 |  |  |  |
Loans held for investment |  | 713.1 |  |  |  |
Allowance for loan loss |  | 0.0 |  |  |  |
Premises and equipment |  | 14.5 |  |  |  |
Other real estate owned (“OREO”) |  | 0.6 |  |  |  |
Core deposit intangible assets |  | 15.7 |  |  |  |
Other assets |  | 19.3 |  |  |  |
Total assets acquired |  | 797.5 |  |  |  |
Deposits |  | 696.3 |  |  |  |
Accounts Payable and Accrued Expense |  | 7.7 |  |  |  |
Long term debt |  | 13.1 |  |  |  |
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 |  |  |  |
Inland Northwest Bank |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Total consideration paid |  | 176.3 |  |  |  |
Goodwill |  | $ 100.7 |  |  |  |
Cascade Bank Acquisition, as Recorded by the Company [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Net increase to goodwill |  |  | $ 0.9 |  |  |
[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] | (6) Adjustment consists of reductions to the fair value of other items, including the removal of Northwest previously recorded goodwill.
[6] | (7) Increase in book value of time deposits to their estimated fair values based upon interest rates of similar time deposits with similar terms on the date of acquisition based upon valuation from an independent accounting and advisory firm.
[7] | (9) Write down of the book value of debt to the estimated fair values on the date of acquisition based upon favorable interest rates in the market.

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- Definition Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460 + Details
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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=109124213&loc=d3e6613-128477 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=109124213&loc=d3e6578-128477 + Details
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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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- Definition Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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- Definition The amount of identifiable intangible assets recognized as of the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=108330185&loc=d3e2207-128464 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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- Definition Amount of liabilities assumed at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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- Definition Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=108330185&loc=d3e2207-128464 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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- Definition The amount of property, plant, and equipment recognized as of the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=108330185&loc=d3e2207-128464 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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