Investments - Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 53.7	[1]	$ 59.2	[1]
Fixed maturities, including securities pledged, Fair Value	21,500.0		19,200.0
OTTI	27.5	[2]	38.1	[2]
Securities pledged, Amortized Cost	207.2		572.5
Equity securities, cost	129.3		131.8
Total fixed maturities and equity securities, Amortized Cost	19,132.7		17,221.6
Gross Unrealized Capital Gains	2,244.5		1,662.8
Gross Unrealized Capital Losses	52.6		152.2
Fair Value	21,378.3		18,791.4
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,011.5		1,096.6
Fixed maturities, Gross Unrealized Capital Gains	135.6		135.0
Fixed maturities, Gross Unrealized Capital Losses	0.5		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	1,146.6		1,231.6
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	379.4		379.7
Fixed maturities, Gross Unrealized Capital Gains	17.6		31.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	397.0		410.7
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		95.1
Fixed maturities, Gross Unrealized Capital Gains	15.9		10.9
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	93.1		106.0
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	9,438.0		8,166.9
Fixed maturities, Gross Unrealized Capital Gains	1,147.4		770.8
Fixed maturities, Gross Unrealized Capital Losses	11.1		31.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	10,574.3		8,906.6
OTTI	2.0	[2]	0	[2]
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,009.7	[3]	4,661.0	[3]
Fixed maturities, Gross Unrealized Capital Gains	558.7	[3]	368.6	[3]
Fixed maturities, Gross Unrealized Capital Losses	16.4	[3]	41.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,552.0	[3]	4,988.1	[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	439.7	[3]	308.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	57.4	[3]	39.8	[3]
Fixed maturities, Gross Unrealized Capital Losses	1.1	[3]	3.1	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	496.0	[3]	345.2	[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	4,570.0	[3]	4,352.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	501.3	[3]	328.8	[3]
Fixed maturities, Gross Unrealized Capital Losses	15.3	[3]	38.4	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,056.0	[3]	4,642.9	[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	2,070.4		1,955.4
Fixed maturities, Gross Unrealized Capital Gains	251.5		285.4
Fixed maturities, Gross Unrealized Capital Losses	18.1		52.9
Embedded Derivatives	53.7	[1]	59.2	[1]
Fixed maturities, including securities pledged, Fair Value	2,357.5		2,247.1
OTTI	18.0	[2]	29.5	[2]
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,679.5		1,442.0
Fixed maturities, Gross Unrealized Capital Gains	181.5		218.7
Fixed maturities, Gross Unrealized Capital Losses	3.4		3.4
Embedded Derivatives	33.7	[1]	39.4	[1]
Fixed maturities, including securities pledged, Fair Value	1,891.3		1,696.7
OTTI	0.6	[2]	0.7	[2]
Residential mortgage-backed securities non-agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	390.9		513.4
Fixed maturities, Gross Unrealized Capital Gains	70.0		66.7
Fixed maturities, Gross Unrealized Capital Losses	14.7		49.5
Embedded Derivatives	20.0	[1]	19.8	[1]
Fixed maturities, including securities pledged, Fair Value	466.2		550.4
OTTI	17.4	[2]	28.8	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	748.7		866.1
Fixed maturities, Gross Unrealized Capital Gains	90.6		51.0
Fixed maturities, Gross Unrealized Capital Losses	0.2		5.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	839.1		911.3
OTTI	4.4	[2]	4.4	[2]
Gross Unrealized Capital Losses	0.2		5.8
Fair Value	839.1		911.3
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	475.7		441.5
Fixed maturities, Gross Unrealized Capital Gains	26.6		19.4
Fixed maturities, Gross Unrealized Capital Losses	6.7		22.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	495.6		438.8
OTTI	3.1	[2]	4.2	[2]
Gross Unrealized Capital Losses	0.6		0.7
Fair Value	435.6		381.0
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	19,210.6		17,662.3
Fixed maturities, Gross Unrealized Capital Gains	2,243.9		1,672.1
Fixed maturities, Gross Unrealized Capital Losses	53.0		153.4
Embedded Derivatives	53.7	[1]	59.2	[1]
Fixed maturities, including securities pledged, Fair Value	21,455.2		19,240.2
OTTI	27.5	[2]	38.1	[2]
Securities pledged, Amortized Cost	207.2		572.5
Securities pledged, Gross Unrealized Capital Gains	13.0		22.4
Securities pledged, Gross Unrealized Capital Losses	0.5		1.2
Securities pledged, Gross Unrealized Capital Losses	219.7		593.7
Total fixed maturities, less securities pledged, Amortized Cost	19,003.4		17,089.8
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	2,230.9		1,649.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	52.5		152.2
Total fixed maturities, less securities pledged, Fair Value	21,235.5		18,646.5
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0	[1]
OTTI	0	[2]	0	[2]
Equity securities, cost	129.3		131.8
Gross Unrealized Capital Gains	13.6		13.1
Gross Unrealized Capital Losses	0.1		0
Fair Value	$ 142.8		$ 144.9

[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) on the 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.