Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financial Assets
Gross Amounts Recognized	$ 49.4	$ 55.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	49.4	55.3
Financial Instruments	17.5	0.0
Fair Value of Financial Collateral in the Balance Sheet	12.4	17.2
Net Amount	19.5	38.1
Financial Liabilities
Gross Amounts Recognized	47.0	53.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	47.0	53.3
Financial Instruments	17.5	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	29.5	53.3
Repurchase agreements
Gross Amounts Recognized	1,052.6	1,091.4
Gross Amounts Offset in the Balance Sheet		0.0
Net Amounts in the Balance Sheet	1,052.6	1,091.4
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	1,052.6	1,091.4
Net Amount	0.0	0.0
Total assets and liabilities
Gross Amounts Recognized	1,099.6	1,144.7
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	1,099.6	1,144.7
Financial Instruments	17.5	0.0
Fair Value of Financial Collateral in the Balance Sheet	1,052.6	1,091.4
Net Amount	29.5	53.3
Interest Rate Swap [Member]
Financial Assets
Gross Amounts Recognized	46.8	52.0
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	46.8	52.0
Financial Instruments	17.5	0.0
Fair Value of Financial Collateral in the Balance Sheet	12.4	17.2
Net Amount	16.9	34.8
Financial Liabilities
Gross Amounts Recognized	47.0	52.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	47.0	52.2
Financial Instruments	17.5	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	29.5	52.2
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	2.6	3.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	2.6	3.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 2.6	3.3
Financial Liabilities
Gross Amounts Recognized		1.1
Gross Amounts Offset in the Balance Sheet		0.0
Net Amounts in the Balance Sheet		1.1
Financial Instruments		0.0
Fair Value of Financial Collateral in the Balance Sheet		0.0
Net Amount		$ 1.1