Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financial Assets
Gross Amounts Recognized	$ 65.1	$ 23.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	65.1	23.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	35.1	18.0
Net Amount	30.0	5.1
Financial Liabilities
Gross Amounts Recognized	59.5	22.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	59.5	22.2
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	59.5	22.1
Repurchase agreements
Gross Amounts Recognized	614.1	697.6
Gross Amounts Offset in the Balance Sheet		0.0
Net Amounts in the Balance Sheet	614.1	697.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	614.1	697.6
Net Amount	0.0	0.0
Total assets and liabilities
Gross Amounts Recognized	673.6	719.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	673.6	719.8
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	614.1	697.6
Net Amount	59.5	22.1
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	57.4	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	57.4	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	35.1	18.0
Net Amount	22.3	3.8
Financial Liabilities
Gross Amounts Recognized	57.4	21.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	57.4	21.9
Financial Instruments	0.0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	57.4	21.8
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	7.7	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	7.7	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	7.7	1.3
Financial Liabilities
Gross Amounts Recognized	2.1	0.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	2.1	0.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 2.1	$ 0.3