Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financial Assets
Gross Amounts Recognized	$ 68.3	$ 23.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	68.3	23.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	28.7	18.0
Net Amount	39.6	5.1
Financial Liabilities
Gross Amounts Recognized	63.1	22.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	63.1	22.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	63.1	22.1
Repurchase agreements
Gross Amounts Recognized	820.3	697.6
Gross Amounts Offset in the Balance Sheet		0.0
Net Amounts in the Balance Sheet	820.3	697.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	820.3	697.6
Net Amount	0.0	0.0
Total assets and liabilities
Gross Amounts Recognized	883.4	719.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	883.4	719.8
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	820.3	697.6
Net Amount	63.1	22.1
Interest Rate Swap [Member]
Financial Assets
Gross Amounts Recognized	61.3	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	61.3	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	28.7	18.0
Net Amount	32.6	3.8
Financial Liabilities
Gross Amounts Recognized	61.5	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	61.5	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	61.5	21.8
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	7.0	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	7.0	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	7.0	1.3
Financial Liabilities
Gross Amounts Recognized	1.6	0.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	1.6	0.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 1.6	$ 0.3