Derivative Financial Instruments Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Derivatives, Asset Fair Value	$ 1,906.7	$ 1,714.9
Derivatives, Liability Fair Value	688.7	841.1
Counterparty netting, Assets	(585.8)	(721.3)
Counterparty netting, Liabilities	(585.8)	(721.3)
Cash collateral netting, Assets	(1,182.9)	(661.1)
Cash collateral netting, Liabilities	(36.8)	(35.9)
Securities collateral netting, Assets	(7.5)	(158.9)
Securities collateral netting, Liabilities	(39.6)	(46.9)
Net receivables/payables, Assets	130.5	173.6
Net receivables/payables, Liabilities	26.5	37.0
Collateralized loan obligations notes, at fair value using the fair value option	7,225.6	6,838.1
Securities pledged as collateral
Offsetting Assets and Liabilities [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	7.8	159.3
Fair value of securities delivered as collateral	662.5	638.7
Credit contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	4,341.3	4,221.0
Derivatives, Asset Fair Value	26.8	40.9
Derivatives, Liability Fair Value	17.3	36.0
Equity contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	16,865.6	13,576.1
Derivatives, Asset Fair Value	388.8	378.4
Derivatives, Liability Fair Value	104.1	201.7
Foreign exchange contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	1,459.3	1,547.8
Derivatives, Asset Fair Value	88.8	70.6
Derivatives, Liability Fair Value	34.6	26.8
Interest rate contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	56,920.5	67,776.4
Derivatives, Asset Fair Value	1,402.3	1,225.0
Derivatives, Liability Fair Value	$ 532.7	$ 576.6