Derivative Financial Instruments - Credit Default Swaps (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Securities pledged as collateral	$ 180.4		$ 219.7
Asset Fair Value	443.6		566.4
Liability Fair Value	182.0		469.2
Payables under securities loan agreement, including collateral held
Derivatives, Fair Value [Line Items]
Cash collateral held for securities loan agreement	120.9		167.0
Credit contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Maximum potential future net exposure on sale of credit default swaps	384.0		329.0
Credit contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	6.2	[1]	3.6	[1]
Liability Fair Value	0	[1]	0	[1]
Maturity period of derivative	5 years
Fixed maturities
Derivatives, Fair Value [Line Items]
Securities pledged as collateral	20.5		39.5
Over the Counter [Member]
Derivatives, Fair Value [Line Items]
Securities pledged as collateral	$ 13.9

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