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Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions |  | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | ---
May 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  |  | $ 445.6 | $ 444.3 | $ 445.6 | $ 444.7 | $ 212.8
Net increase to goodwill |  |  | $ 0.9 | $ 0.0 |  |  |
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 loss |  | (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 |  |  |  |  |
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 | [1] | 4.9 |  |  |  |  |
Loans held for investment | [2] | (31.7) |  |  |  |  |
Mortgage loans held for sale |  | 0.0 |  |  |  |  |
Allowance for loan loss | [3] | 24.0 |  |  |  |  |
Premises and equipment | [4] | 0.1 |  |  |  |  |
Other real estate owned (OREO) |  | 0.0 |  |  |  |  |
Core deposit intangible assets | [5] | 48.0 |  |  |  |  |
Deferred tax assets, net | [6] | (20.9) |  |  |  |  |
Other assets |  | 2.1 |  |  |  |  |
Total assets acquired |  | 26.5 |  |  |  |  |
Deposits | [7] | (0.9) |  |  |  |  |
Accounts Payable and Accrued Expense | [8] | 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 loss |  | 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 |  |  |  |  |
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 |  |  |  |  |
Class A common stock |  | 386.0 |  |  |  |  |
Total consideration paid |  | 541.0 |  |  |  |  |
Goodwill |  | $ 232.8 |  |  |  |  |
Net increase to goodwill |  |  |  |  | $ 0.9 |  |
[1] | Write up of the book value of investments to their estimated fair values on the date of acquisition based upon quotes obtained from an independent third party pricing service.
[2] | Write down of the book value of loans to their estimated fair values. Shared National Credits (SNC) were recorded at quoted sales prices where available. The fair value of the remaining 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.
[3] | Adjustment to remove the Cascade allowance for loan losses at acquisition date, as the credit risk is accounted for in the fair value adjustment for loans receivable described in (2) above.
[4] | Write up of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon appraisals obtained from an independent third party appraiser or broker's opinion of value.
[5] | Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm.
[6] | Adjustment consists of the write-off of pre-existing deferred tax assets as a result of the acquisition.
[7] | Decrease 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.
[8] | Increase in fair value due to credit card incentive program, unfavorable leases, write-off of balance sheet reserve, and swap liability offset.

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