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Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions |  | 3 Months Ended |  |
--- | --- | --- | --- | ---
May 30, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Business Acquisition [Line Items] |  |  |  |  |
Goodwill |  | $ 445.6 | $ 212.8 | $ 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 | 4.9 |  |  |  |
Loans held for investment | (31.7) |  |  |  |
Mortgage loans held for sale | 0.0 |  |  |  |
Allowance for loan loss | 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) |  |  |  |
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 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 |  |  |

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