Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financial Assets
Gross Amounts Recognized	$ 10.1	$ 8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	10.1	8.8
Financial Instruments	2.7	2.4
Fair Value of Financial Collateral in the Balance Sheet	2.4	0.0
Net Amount	5.0	6.4
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	9.4	7.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	9.4	7.9
Financial Instruments	2.7	2.4
Fair Value of Financial Collateral in the Balance Sheet	4.1	3.3
Net Amount	2.6	2.2
Repurchase agreements
Gross Amounts Recognized	712.4	643.0
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	712.4	643.0
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	712.4	643.0
Net Amount	0.0	0.0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	721.8	650.9
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	721.8	650.9
Financial Instruments	2.7	2.4
Fair Value of Financial Collateral in the Balance Sheet	716.5	646.3
Net Amount	2.6	2.2
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	8.8	7.5
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	8.8	7.5
Financial Instruments	2.7	2.4
Fair Value of Financial Collateral in the Balance Sheet	2.4	0.0
Net Amount	3.7	5.1
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	8.8	7.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	8.8	7.8
Financial Instruments	2.7	2.4
Fair Value of Financial Collateral in the Balance Sheet	4.1	3.3
Net Amount	2.0	2.1
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	1.3	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	1.3	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	1.3	1.3
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	0.6	0.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	0.6	0.1
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 0.6	$ 0.1