Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financial Assets
Gross Amounts Recognized	$ 23.2	$ 10.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	23.2	10.1
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	18.0	2.4
Net Amount	5.1	5.0
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	22.2	9.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	22.2	9.4
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.0	4.1
Net Amount	22.1	2.6
Repurchase agreements
Gross Amounts Recognized	697.6	712.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	697.6	712.4
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	697.6	712.4
Net Amount	0.0	0.0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	719.8	721.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	719.8	721.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	697.6	716.5
Net Amount	22.1	2.6
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	21.9	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	21.9	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	18.0	2.4
Net Amount	3.8	3.7
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	21.9	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	21.9	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.0	4.1
Net Amount	21.8	2.0
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	1.3	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	1.3	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	1.3	1.3
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	0.3	0.6
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	0.3	0.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 0.3	$ 0.6