Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial Assets
Gross Amounts Recognized	$ 55.3	$ 23.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	55.3	23.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	17.2	18.0
Net Amount	38.1	5.1
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	53.3	22.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	53.3	22.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	53.3	22.1
Repurchase agreements
Gross Amounts Recognized	1,091.4	697.6
Gross Amounts Offset in the Balance Sheet		0.0
Net Amounts in the Balance Sheet	1,091.4	697.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	1,091.4	697.6
Net Amount	0.0	0.0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	1,144.7	719.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	1,144.7	719.8
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	1,091.4	697.6
Net Amount	53.3	22.1
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	52.0	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	52.0	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	17.2	18.0
Net Amount	34.8	3.8
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	52.2	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	52.2	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	52.2	21.8
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	3.3	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	3.3	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	3.3	1.3
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	1.1	0.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	1.1	0.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 1.1	$ 0.3