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Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Derivative [Line Items] |  |
Derivative Asset, Gross Amounts Recognized | $ 12.4 | $ 8.8
Derivative Asset, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amounts in the Balance Sheet | 12.4 | 8.8
Derivative Asset, Financial Instruments | 1.0 | 2.4
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet | 1.8 | 0.0
Derivative Asset, Net Amount | 9.6 | 6.4
Total assets, Gross Amounts Recognized | 12.4 | 8.8
Total assets, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Total assets, Net Amounts in the Balance Sheet | 12.4 | 8.8
Total assets, Financial Instruments | 1.0 | 2.4
Total assets, Fair Value of Financial Collateral in the Balance Sheet | 1.8 | 0.0
Total assets, Net Amount | 9.6 | 6.4
Derivative liability, Gross Amounts Recognized | 11.3 | 7.9
Derivative liability, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative liability, Net Amounts in the Balance Sheet | 11.3 | 7.9
Derivative liability, Financial Instruments | 1.0 | 2.4
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 10.0 | 3.3
Derivative liability, Net Amount | 0.3 | 2.2
Repurchase agreements, Gross Amounts Recognized | 635.9 | 643.0
Repurchase agreements, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Repurchase agreements, Net Amounts in the Balance Sheet | 635.9 | 643.0
Repurchase agreements, Financial Instruments | 0.0 | 0.0
Repurchase agreements, Fair Value of Financial Collateral in the Balance Sheet | 635.9 | 643.0
Repurchase agreements, Net Amount | 0.0 | 0.0
Total liabilities, Gross Amounts Recognized | 647.2 | 650.9
Total liabilities, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Total liabilities, Net Amounts in the Balance Sheet | 647.2 | 650.9
Total liabilities, Financial Instruments | 1.0 | 2.4
Total liabilities, Fair Value of Financial Collateral in the Balance Sheet | 645.9 | 646.3
Total liabilities, Net Amount | 0.3 | 2.2
Interest Rate Swap |  |
Derivative [Line Items] |  |
Derivative Asset, Gross Amounts Recognized | 11.3 | 7.5
Derivative Asset, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amounts in the Balance Sheet | 11.3 | 7.5
Derivative Asset, Financial Instruments | 1.0 | 2.4
Derivative Asset, Fair Value of Financial Collateral in the Balance Sheet | 1.8 | 0.0
Derivative Asset, Net Amount | 8.5 | 5.1
Derivative liability, Gross Amounts Recognized | 11.1 | 7.8
Derivative liability, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative liability, Net Amounts in the Balance Sheet | 11.1 | 7.8
Derivative liability, Financial Instruments | 1.0 | 2.4
Derivative liability, Fair Value of Financial Collateral in the Balance Sheet | 10.0 | 3.3
Derivative liability, Net Amount | 0.1 | 2.1
Mortgage Related Derivatives |  |
Derivative [Line Items] |  |
Derivative Asset, Gross Amounts Recognized | 1.1 | 1.3
Derivative Asset, Gross Amounts Offset in the Balance Sheet | 0.0 | 0.0
Derivative Asset, Net Amounts in the Balance Sheet | 1.1 | 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 | 1.1 | 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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- Definition Amount of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price, elected not to be offset, deducted from securities sold under agreements to repurchase. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(1)(i) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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