Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 164.0	$ 470.8
Liability Fair Value	39.5	172.3
Counterparty netting, Assets	(35.6)	(169.0)
Counterparty netting, Liabilities	(35.6)	(169.0)
Cash collateral netting, Assets	(114.7)	(265.8)
Cash collateral netting, Liabilities	0.0	0.0
Securities collateral netting, Assets	(9.2)	(11.0)
Securities collateral netting, Liabilities	(3.5)	(1.7)
Net receivables	4.5	25.0
Net payables	0.4	1.6
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	543.3	425.5
Asset Fair Value	5.5	5.6
Liability Fair Value	2.7	0.2
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	107.5	89.4
Asset Fair Value	3.2	2.3
Liability Fair Value	0.7	0.0
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	419.6	405.9
Asset Fair Value	10.0	25.4
Liability Fair Value	14.1	18.3
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	22,941.7	25,606.7
Asset Fair Value	145.3	437.5
Liability Fair Value	$ 22.0	$ 153.8