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Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Derivative [Line Items] |  |
Derivative Asset, Gross Amounts Recognized | $ 9.7 | $ 8.8
Derivative Asset, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amounts in the Balance Sheet | 9.7 | 8.8
Derivative Asset, Financial Instruments | 1.7 | 2.4
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet | 1.8 | 0.0
Derivative Asset, Net Amount | 6.2 | 6.4
Total assets, Gross Amounts Recognized | 9.7 | 8.8
Total assets, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Total assets, Net Amounts in the Balance Sheet | 9.7 | 8.8
Total assets, Financial Instruments | 1.7 | 2.4
Total assets, Fair Value of Financial Collateral in the Balance Sheet | 1.8 | 0.0
Total assets, Net Amount | 6.2 | 6.4
Derivative liability, Gross Amounts Recognized | 7.9 | 7.9
Derivative liability, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative liability, Net Amounts in the Balance Sheet | 7.9 | 7.9
Derivative liability, Financial Instruments | 1.7 | 2.4
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 5.7 | 3.3
Derivative liability, Net Amount | 0.5 | 2.2
Repurchase agreements, Gross Amounts Recognized | 633.8 | 643.0
Repurchase agreements, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Repurchase agreements, Net Amounts in the Balance Sheet | 633.8 | 643.0
Repurchase agreements, Financial Instruments | 0.0 | 0.0
Repurchase agreements, Fair Value of Financial Collateral in the Balance Sheet | 633.8 | 643.0
Repurchase agreements, Net Amount | 0.0 | 0.0
Total liabilities, Gross Amounts Recognized | 641.7 | 650.9
Total liabilities, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Total liabilities, Net Amounts in the Balance Sheet | 641.7 | 650.9
Total liabilities, Financial Instruments | 1.7 | 2.4
Total liabilities, Fair Value of Financial Collateral in the Balance Sheet | 639.5 | 646.3
Total liabilities, Net Amount | 0.5 | 2.2
Interest Rate Swap |  |
Derivative [Line Items] |  |
Derivative Asset, Gross Amounts Recognized | 8.3 | 7.5
Derivative Asset, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amounts in the Balance Sheet | 8.3 | 7.5
Derivative Asset, Financial Instruments | 1.7 | 2.4
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet | 1.8 | 0.0
Derivative Asset, Net Amount | 4.8 | 5.1
Derivative liability, Gross Amounts Recognized | 7.7 | 7.8
Derivative liability, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative liability, Net Amounts in the Balance Sheet | 7.7 | 7.8
Derivative liability, Financial Instruments | 1.7 | 2.4
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 5.7 | 3.3
Derivative liability, Net Amount | 0.3 | 2.1
Mortgage Related Derivatives |  |
Derivative [Line Items] |  |
Derivative Asset, Gross Amounts Recognized | 1.4 | 1.3
Derivative Asset, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amounts in the Balance Sheet | 1.4 | 1.3
Derivative Asset, Financial Instruments | 0.0 | 0.0
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amount | 1.4 | 1.3
Derivative liability, Gross Amounts Recognized | 0.2 | 0.1
Derivative liability, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative liability, Net Amounts in the Balance Sheet | 0.2 | 0.1
Derivative liability, Financial Instruments | 0.0 | 0.0
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 0.0 | 0.0
Derivative liability, Net Amount | $ 0.2 | $ 0.1

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