Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financial Assets
Gross Amounts Recognized	$ 14.6	$ 10.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Derivative assets:	14.6	10.1
Financial Instruments	2.0	2.7
Fair Value of Financial Collateral in the Balance Sheet	7.5	2.4
Net Amount	5.1	5.0
Financial Liabilities
Gross Amounts Recognized	13.5	9.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Derivative liabilities:	13.5	9.4
Financial Instruments	2.0	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.2	4.1
Net Amount	11.3	2.6
Repurchase agreements
Gross Amounts Recognized	672.6	712.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	672.6	712.4
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	672.6	712.4
Net Amount	0.0	0.0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned
Gross Amounts Recognized	686.1	721.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	686.1	721.8
Financial Instruments	2.0	2.7
Fair Value of Financial Collateral in the Balance Sheet	672.8	716.5
Net Amount	11.3	2.6
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	12.7	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Derivative assets:	12.7	8.8
Financial Instruments	2.0	2.7
Fair Value of Financial Collateral in the Balance Sheet	7.5	2.4
Net Amount	3.2	3.7
Financial Liabilities
Gross Amounts Recognized	12.7	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Derivative liabilities:	12.7	8.8
Financial Instruments	2.0	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.2	4.1
Net Amount	10.5	2.0
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	1.9	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Derivative assets:	1.9	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	1.9	1.3
Financial Liabilities
Gross Amounts Recognized	0.8	0.6
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Derivative liabilities:	0.8	0.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 0.8	$ 0.6