Derivative Financial Instruments - Collateral and Credit Default Swaps (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	$ 6,344.4		$ 5,161.6
Fair value of credit default swaps included in Derivatives assets	1,255.0		1,275.8
Fair value of credit default swaps included in Derivatives liabilities	3,876.7		4,076.4
Securities pledged as collateral
Derivatives, Fair Value [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	71.2		51.3
Credit contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Maximum potential future net exposure on sale of credit default swaps	1,700.0		1,700.0
Purchased protection on credit default swaps	500		500
Maturity period of derivative	5 years		5 years
Credit contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Fair value of credit default swaps included in Derivatives assets	41.4	[1]	40.5	[1]
Fair value of credit default swaps included in Derivatives liabilities	24.0	[1]	14.5	[1]
Over the counter | Cash collateral, included in Payables
Derivatives, Fair Value [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	298.6		214.7
Cleared derivative contract | Cash collateral, included in Payables
Derivatives, Fair Value [Line Items]
Collateralized loan obligations notes, at fair value using the fair value option	$ 51.6		$ 18.8

[1] Open derivative contracts are reported as Derivatives assets or liabilities on the Condensed Consolidated Balance Sheets at fair value.