Investments Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 58.9	[1]	$ 59.2	[1]
Fixed maturities, including securities pledged, Fair Value	19,900.0
OTTI	34.5	[2]	38.1	[2]
Securities pledged, Amortized Cost	342.2		572.5
Equity securities, cost	131.2		131.8
Total fixed maturities and equity securities, Amortized Cost	17,814.2		17,221.6
Gross Unrealized Capital Gains	1,891.9		1,662.8
Gross Unrealized Capital Losses	102.2		152.2
Fair Value	19,662.8		18,791.4
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	903.8		1,096.6
Fixed maturities, Gross Unrealized Captial Gains	149.7		135.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	1,053.5		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	378.9		379.7
Fixed maturities, Gross Unrealized Captial Gains	24.5		31.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	403.4		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 Captial Gains	13.3		10.9
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	90.5		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	8,692.1		8,166.9
Fixed maturities, Gross Unrealized Captial Gains	922.1		770.8
Fixed maturities, Gross Unrealized Capital Losses	23.9		31.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	9,590.3		8,906.6
OTTI	0	[2]	0	[2]
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	4,727.5	[3]	4,661.0	[3]
Fixed maturities, Gross Unrealized Captial Gains	424.0	[3]	368.6	[3]
Fixed maturities, Gross Unrealized Capital Losses	23.0	[3]	41.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,128.5	[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	419.3	[3]	308.5	[3]
Fixed maturities, Gross Unrealized Captial Gains	35.5	[3]	39.8	[3]
Fixed maturities, Gross Unrealized Capital Losses	1.4	[3]	3.1	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	453.4	[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	4,308.2	[3]	4,352.5	[3]
Fixed maturities, Gross Unrealized Captial Gains	388.5	[3]	328.8	[3]
Fixed maturities, Gross Unrealized Capital Losses	21.6	[3]	38.4	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	4,675.1	[3]	4,642.9	[3]
OTTI	0	[2],[3]	0	[2],[3]
Total residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,966.0		1,955.4
Fixed maturities, Gross Unrealized Captial Gains	271.1		285.4
Fixed maturities, Gross Unrealized Capital Losses	38.8		52.9
Embedded Derivatives	58.9	[1]	59.2	[1]
Fixed maturities, including securities pledged, Fair Value	2,257.2		2,247.1
OTTI	26.2	[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	1,513.8		1,442.0
Fixed maturities, Gross Unrealized Captial Gains	202.9		218.7
Fixed maturities, Gross Unrealized Capital Losses	4.6		3.4
Embedded Derivatives	37.9	[1]	39.4	[1]
Fixed maturities, including securities pledged, Fair Value	1,750.0		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	452.2		513.4
Fixed maturities, Gross Unrealized Captial Gains	68.2		66.7
Fixed maturities, Gross Unrealized Capital Losses	34.2		49.5
Embedded Derivatives	21.0	[1]	19.8	[1]
Fixed maturities, including securities pledged, Fair Value	507.2		550.4
OTTI	25.6	[2]	28.8	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	828.4		866.1
Fixed maturities, Gross Unrealized Captial Gains	64.9		51.0
Fixed maturities, Gross Unrealized Capital Losses	1.4		5.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	891.9		911.3
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	451.3		441.5
Fixed maturities, Gross Unrealized Captial Gains	22.3		19.4
Fixed maturities, Gross Unrealized Capital Losses	17.2		22.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	456.4		438.8
OTTI	3.9	[2]	4.2	[2]
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	18,025.2		17,662.3
Fixed maturities, Gross Unrealized Captial Gains	1,891.9		1,672.1
Fixed maturities, Gross Unrealized Capital Losses	104.3		153.4
Embedded Derivatives	58.9	[1]	59.2	[1]
Fixed maturities, including securities pledged, Fair Value	19,871.7		19,240.2
OTTI	34.5	[2]	38.1	[2]
Securities pledged, Amortized Cost	342.2		572.5
Securities pledged, Gross Unrealized Capital Gains	13.5		22.4
Securities pledged, Gross Unrealized Capital Losses	2.1		1.2
Securities pledged, Gross Unrealized Capital Losses	353.6		593.7
Total fixed maturities, less securities pledged, Amortized Cost	17,683.0		17,089.8
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,878.4		1,649.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	102.2		152.2
Total fixed maturities, less securities pledged, Fair Value	19,518.1		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	131.2		131.8
Gross Unrealized Capital Gains	13.5		13.1
Gross Unrealized Capital Losses	0		0
Fair Value	$ 144.7		$ 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) 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.