Derivative Financial Instruments - Credit Default Swaps (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Cash collateral held for securities loan agreement	$ 0		$ 16.3
Securities pledged as collateral	236.4		140.1
Asset Fair Value	473.5		493.4
Liability Fair Value	239.0		185.7
Payables under securities loan agreement, including collateral held
Derivatives, Fair Value [Line Items]
Cash collateral held for securities loan agreement	172.5		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
Maturity period of derivative	5 years
Credit contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	7.5	[1]	8.1	[1]
Liability Fair Value	0	[1]	0	[1]
Fixed maturities
Derivatives, Fair Value [Line Items]
Collateral Securities Repledged, Delivered, or Used	59.1		42.5
Over the Counter [Member]
Derivatives, Fair Value [Line Items]
Securities pledged as collateral	$ 58.1		$ 1.2

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