Derivative Financial Instruments Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Offsetting Assets and Liabilities [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	$ 6,344.4		$ 5,161.6
Derivatives, Asset Fair Value	1,120.0		1,147.3
Derivatives, Liability Fair Value	655.1		1,330.9
Counterparty netting, Assets	(505.5)	[1]	(701.0)	[1]
Counterparty netting, Liabilities	(505.5)	[1]	(701.0)	[1]
Cash collateral netting, Assets	(383.4)	[1]	(241.7)	[1]
Cash collateral netting, Liabilities	(29.6)	[1]	(15.7)	[1]
Securities collateral netting, Assets	(66.7)	[1]	(35.9)	[1]
Securities collateral netting, Liabilities	(65.6)	[1]	(479.1)	[1]
Net receivables/payables, Assets	164.4		168.7
Net receivables/payables, Liabilities	54.4		135.1
Credit contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	3,221.0		3,221.0
Derivatives, Asset Fair Value	41.4		40.5
Derivatives, Liability Fair Value	24.0		14.5
Equity contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	9,928.6		4,513.5
Derivatives, Asset Fair Value	234.9		170.7
Derivatives, Liability Fair Value	40.5		32.0
Foreign exchange contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	1,466.9		1,456.4
Derivatives, Asset Fair Value	47.5		24.1
Derivatives, Liability Fair Value	20.5		44.9
Interest rate contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	70,393.0		64,734.1
Derivatives, Asset Fair Value	796.2		912.0
Derivatives, Liability Fair Value	570.1		1,239.5
Securities pledged as collateral
Offsetting Assets and Liabilities [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	71.2		51.3
Fair value of securities delivered as collateral	$ 611.7		$ 1,000.0

[1] Represents the netting of receivable balances with payable balances, net of collateral, for the same counterparty under eligible netting agreements.