Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
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