Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial Assets
Gross Amounts Recognized	$ 24.5	$ 10.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	24.5	10.1
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	14.9	2.4
Net Amount	9.5	5.0
Financial Liabilities
Gross Amounts Recognized	22.7	9.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	22.7	9.4
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.0	4.1
Net Amount	22.6	2.6
Repurchase agreements
Gross Amounts Recognized	686.7	712.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	686.7	712.4
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	686.7	712.4
Net Amount	0.0	0.0
Total assets and liabilities
Gross Amounts Recognized	709.4	721.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	709.4	721.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	686.7	716.5
Net Amount	22.6	2.6
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	22.0	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	22.0	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	14.9	2.4
Net Amount	7.0	3.7
Financial Liabilities
Gross Amounts Recognized	22.0	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	22.0	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.0	4.1
Net Amount	21.9	2.0
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	2.5	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	2.5	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	2.5	1.3
Financial Liabilities
Gross Amounts Recognized	0.7	0.6
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	0.7	0.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 0.7	$ 0.6