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