Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 594.1	$ 450.3
Liability Fair Value	235.7	114.4
Counterparty netting, Assets	(171.6)	(111.7)
Counterparty netting, Liabilities	(171.6)	(111.7)
Cash collateral netting, Assets	(383.8)	(298.0)
Cash collateral netting, Liabilities	(54.7)	(0.3)
Securities collateral netting, Assets	(11.9)	(11.0)
Securities collateral netting, Liabilities	(6.5)	(2.4)
Net receivables	26.8	29.6
Net payables	2.9	0.0
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	557.5	407.5
Asset Fair Value	4.0	3.3
Liability Fair Value	0.3	0.3
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	91.6	0.0
Asset Fair Value	2.3	0.0
Liability Fair Value	0.0	0.0
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	264.6	195.8
Asset Fair Value	14.0	24.6
Liability Fair Value	9.3	10.7
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	23,897.6	22,965.5
Asset Fair Value	573.8	422.4
Liability Fair Value	$ 226.1	$ 103.4