Derivative Financial Instruments - Credit Default Swaps (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Securities pledged as collateral	$ 211.8		$ 140.1
Asset Fair Value	441.4		493.4
Liability Fair Value	190.6		185.7
Payables under securities loan agreement, including collateral held
Derivatives, Fair Value [Line Items]
Cash collateral held for securities loan agreement	166.4		127.4
Credit contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	384.0		384.0
Credit contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	384.0	[1]	384.0	[1]
Maximum potential future net exposure on sale of credit default swaps	384.0		384.0
Credit contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	7.4	[1]	8.1	[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	53.8		42.5
Over the Counter [Member]
Derivatives, Fair Value [Line Items]
Securities pledged as collateral	$ 31.7		$ 1.2

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