Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 397.1		$ 512.3
Liability Fair Value	200.3		346.8
Counterparty netting, Assets	(187.9)	[1]	(291.4)	[1]
Counterparty netting, Liabilities	(187.9)	[1]	(291.4)	[1]
Cash collateral netting, Assets	(131.4)	[2]	(167.1)	[2]
Cash collateral netting, Liabilities	0	[2]	0	[2]
Securities collateral netting, Assets	(19.2)	[2]	(3.1)	[2]
Securities collateral netting, Liabilities	(7.4)	[2]	(35.8)	[2]
Net receivables	58.6		50.7
Net payables	5.0		19.6
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	384.0		347.5
Asset Fair Value	6.2		3.6
Liability Fair Value	0		0
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	0		0
Asset Fair Value	0		0
Liability Fair Value	0		0
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	225.4		161.6
Asset Fair Value	4.3		0.4
Liability Fair Value	15.0		18.3
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	17,006.4		19,131.1
Asset Fair Value	386.6		508.3
Liability Fair Value	$ 185.3		$ 328.5

[1] Represents the netting of receivable balances with payable balances for the same counterparty under enforceable netting agreements.

[2] Represents the netting of collateral received and posted on a counterparty basis under credit support agreements.