Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Financial Assets
Gross Amounts Recognized	$ 72.3	$ 23.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	72.3	23.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	32.7	18.0
Net Amount	39.6	5.1
Financial Liabilities
Gross Amounts Recognized	66.4	22.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	66.4	22.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	66.4	22.1
Repurchase agreements
Gross Amounts Recognized	756.1	697.6
Gross Amounts Offset in the Balance Sheet		0.0
Net Amounts in the Balance Sheet	756.1	697.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	756.1	697.6
Net Amount	0.0	0.0
Total assets and liabilities
Gross Amounts Recognized	822.5	719.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	822.5	719.8
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	756.1	697.6
Net Amount	66.4	22.1
Interest Rate Swap [Member]
Financial Assets
Gross Amounts Recognized	62.5	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	62.5	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	32.7	18.0
Net Amount	29.8	3.8
Financial Liabilities
Gross Amounts Recognized	62.7	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	62.7	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	62.7	21.8
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	9.8	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	9.8	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	9.8	1.3
Financial Liabilities
Gross Amounts Recognized	3.7	0.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	3.7	0.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 3.7	$ 0.3