Investments - Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 32.9	[1]	$ 53.7	[1]
Equity securities, available-for-sale	127.3		142.8
OTTI	24.1	[2]	27.5	[2]
Securities pledged, Amortized Cost	178.0		207.2
Equity securities, cost	111.2		129.3
Total fixed maturities and equity securities, Amortized Cost	19,734.0		19,132.7
Gross Unrealized Capital Gains	1,252.5		2,244.5
Gross Unrealized Capital Losses	353.4		52.6
Fair Value	20,666.0		21,378.3
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	745.0		1,011.5
Fixed maturities, Gross Unrealized Capital Gains	58.1		135.6
Fixed maturities, Gross Unrealized Capital Losses	2.4		0.5
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	800.7		1,146.6
OTTI	0	[2]	0	[2]
U.S. government agencies and authorities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	242.9		379.4
Fixed maturities, Gross Unrealized Capital Gains	7.4		17.6
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	250.3		397.0
OTTI	0	[2]	0	[2]
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	77.2		77.2
Fixed maturities, Gross Unrealized Capital Gains	6.8		15.9
Fixed maturities, Gross Unrealized Capital Losses	0.1		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	83.9		93.1
OTTI	0	[2]	0	[2]
U.S. corporate securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	10,210.0		9,438.0
Fixed maturities, Gross Unrealized Capital Gains	584.6		1,147.4
Fixed maturities, Gross Unrealized Capital Losses	225.8		11.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	10,568.8		10,574.3
OTTI	1.9	[2]	2.0	[2]
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,504.0	[3]	5,009.7	[3]
Fixed maturities, Gross Unrealized Capital Gains	313.4	[3]	558.7	[3]
Fixed maturities, Gross Unrealized Capital Losses	107.1	[3]	16.4	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,710.3	[3]	5,552.0	[3]
OTTI	0	[2],[3]	0	[2],[3]
Foreign securities government
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	426.8	[3]	439.7	[3]
Fixed maturities, Gross Unrealized Capital Gains	27.0	[3]	57.4	[3]
Fixed maturities, Gross Unrealized Capital Losses	16.2	[3]	1.1	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	437.6	[3]	496.0	[3]
OTTI	0	[2],[3]	0	[2],[3]
Foreign securities other
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,077.2	[3]	4,570.0	[3]
Fixed maturities, Gross Unrealized Capital Gains	286.4	[3]	501.3	[3]
Fixed maturities, Gross Unrealized Capital Losses	90.9	[3]	15.3	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,272.7	[3]	5,056.0	[3]
OTTI	0	[2],[3]	0	[2],[3]
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,931.0		2,070.4
Fixed maturities, Gross Unrealized Capital Gains	184.4		251.5
Fixed maturities, Gross Unrealized Capital Losses	21.3		18.1
Embedded Derivatives	32.9	[1]	53.7	[1]
Fixed maturities, including securities pledged, Fair Value	2,127.0		2,357.5
OTTI	14.6	[2]	18.0	[2]
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,628.1		1,679.5
Fixed maturities, Gross Unrealized Capital Gains	128.6		181.5
Fixed maturities, Gross Unrealized Capital Losses	12.7		3.4
Embedded Derivatives	19.3	[1]	33.7	[1]
Fixed maturities, including securities pledged, Fair Value	1,763.3		1,891.3
OTTI	0.5	[2]	0.6	[2]
Residential mortgage-backed securities non-agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	302.9		390.9
Fixed maturities, Gross Unrealized Capital Gains	55.8		70.0
Fixed maturities, Gross Unrealized Capital Losses	8.6		14.7
Embedded Derivatives	13.6	[1]	20.0	[1]
Fixed maturities, including securities pledged, Fair Value	363.7		466.2
OTTI	14.1	[2]	17.4	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	607.8		748.7
Fixed maturities, Gross Unrealized Capital Gains	71.4		90.6
Fixed maturities, Gross Unrealized Capital Losses	0		0.2
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	679.2		839.1
OTTI	4.4	[2]	4.4	[2]
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	482.9		475.7
Fixed maturities, Gross Unrealized Capital Gains	19.7		26.6
Fixed maturities, Gross Unrealized Capital Losses	3.7		6.7
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	498.9		495.6
OTTI	3.2	[2]	3.1	[2]
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	19,800.8		19,210.6
Fixed maturities, Gross Unrealized Capital Gains	1,245.8		2,243.9
Fixed maturities, Gross Unrealized Capital Losses	360.4		53.0
Embedded Derivatives	32.9	[1]	53.7	[1]
Fixed maturities, including securities pledged, Fair Value	20,719.1		21,455.2
OTTI	24.1	[2]	27.5	[2]
Securities pledged, Amortized Cost	178.0		207.2
Securities pledged, Gross Unrealized Capital Gains	9.4		13.0
Securities pledged, Gross Unrealized Capital Losses	7.0		0.5
Securities pledged, Gross Unrealized Capital Losses	180.4		219.7
Total fixed maturities, less securities pledged, Amortized Cost	19,622.8		19,003.4
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,236.4		2,230.9
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	353.4		52.5
Total fixed maturities, less securities pledged, Fair Value	20,538.7		21,235.5
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0	[1]
Equity securities, available-for-sale	127.3		142.8
OTTI	0	[2]	0	[2]
Equity securities, cost	111.2		129.3
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	16.1		13.6
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	$ 0		$ 0.1

[1] Embedded derivatives within fixed maturity securities are reported with the host investment. The changes in fair value of embedded derivatives are reported in Other net realized capital gains (losses) in the Condensed Consolidated Statements of Operations.

[2] Represents Other-than Temporary-Impairments ("OTTI") reported as a component of Other comprehensive income.

[3] Primarily U.S. dollar denominated.