Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 134.2	$ 470.8
Liability Fair Value	61.6	172.3
Counterparty netting, Assets	(39.3)	(169.0)
Counterparty netting, Liabilities	(39.3)	(169.0)
Cash collateral netting, Assets	(88.6)	(265.8)
Cash collateral netting, Liabilities	0.0	0.0
Securities collateral netting, Assets	(3.1)	(11.0)
Securities collateral netting, Liabilities	(20.1)	(1.7)
Net receivables	3.2	25.0
Net payables	2.2	1.6
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	834.7	425.5
Asset Fair Value	9.6	5.6
Liability Fair Value	6.0	0.2
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	147.1	89.4
Asset Fair Value	5.5	2.3
Liability Fair Value	3.5	0.0
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	482.3	405.9
Asset Fair Value	0.0	25.4
Liability Fair Value	35.6	18.3
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	21,703.9	25,606.7
Asset Fair Value	119.1	437.5
Liability Fair Value	$ 16.5	$ 153.8