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Derivatives and Hedging Activities - Schedule of Notional Amounts of Outstanding Derivative Positions (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Derivative [Line Items] |  |
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | $ 10.0 | $ 3.3
Derivative Asset, Notional Amount | 449.7 | 404.9
Derivative Asset, Estimated Fair Value | 12.4 | 8.8
Notional amount of terminated commitment | 464.2 | 433.0
Derivative Liabilities, Estimated Fair Value | 11.3 | 7.9
Interest rate swap contracts |  |
Derivative [Line Items] |  |
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 10.0 | 3.3
Mortgage Related Derivatives |  |
Derivative [Line Items] |  |
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 0.0 | 0.0
Not Designated as Hedging Instrument | Interest rate swap contracts |  |
Derivative [Line Items] |  |
Derivative Asset, Notional Amount | 386.8 | 344.2
Derivative Asset, Estimated Fair Value | 11.3 | 7.5
Notional amount of terminated commitment | 386.8 | 344.2
Derivative Liabilities, Estimated Fair Value | 11.1 | 7.8
Not Designated as Hedging Instrument | Interest rate lock commitments |  |
Derivative [Line Items] |  |
Derivative Asset, Notional Amount | 62.9 | 60.7
Derivative Asset, Estimated Fair Value | 1.1 | 1.3
Not Designated as Hedging Instrument | Forward loan sales contracts |  |
Derivative [Line Items] |  |
Notional amount of terminated commitment | 77.4 | 88.8
Derivative Liabilities, Estimated Fair Value | $ 0.2 | $ 0.1

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