Investments Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 54.9	[1]	$ 59.2	[1]
Fixed maturities, including securities pledged, Fair Value	20,900.0
OTTI	27.5	[2]	38.1	[2]
Securities pledged, Amortized Cost	201.5		572.5
Equity securities, cost	130.1		131.8
Total fixed maturities and equity securities, Amortized Cost	18,555.2		17,221.6
Gross Unrealized Capital Gains	2,283.1		1,662.8
Gross Unrealized Capital Losses	61.6		152.2
Fair Value	20,831.6		18,791.4
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	927.8		1,096.6
Fixed maturities, Gross Unrealized Capital Gains	148.2		135.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	1,076.0		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	379.2		379.7
Fixed maturities, Gross Unrealized Capital Gains	21.5		31.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	400.7		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	14.1		10.9
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	91.3		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	8,901.7		8,166.9
Fixed maturities, Gross Unrealized Capital Gains	1,169.8		770.8
Fixed maturities, Gross Unrealized Capital Losses	11.0		31.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	10,060.5		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	4,863.2	[3]	4,661.0	[3]
Fixed maturities, Gross Unrealized Capital Gains	548.3	[3]	368.6	[3]
Fixed maturities, Gross Unrealized Capital Losses	19.7	[3]	41.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,391.8	[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	435.7	[3]	308.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	50.7	[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	485.3	[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	4,427.5	[3]	4,352.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	497.6	[3]	328.8	[3]
Fixed maturities, Gross Unrealized Capital Losses	18.6	[3]	38.4	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	4,906.5	[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	2,213.0		1,955.4
Fixed maturities, Gross Unrealized Capital Gains	271.9		285.4
Fixed maturities, Gross Unrealized Capital Losses	22.1		52.9
Embedded Derivatives	54.9	[1]	59.2	[1]
Fixed maturities, including securities pledged, Fair Value	2,517.7		2,247.1
OTTI	19.6	[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	1,794.7		1,442.0
Fixed maturities, Gross Unrealized Capital Gains	200.1		218.7
Fixed maturities, Gross Unrealized Capital Losses	3.6		3.4
Embedded Derivatives	34.0	[1]	39.4	[1]
Fixed maturities, including securities pledged, Fair Value	2,025.2		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	418.3		513.4
Fixed maturities, Gross Unrealized Capital Gains	71.8		66.7
Fixed maturities, Gross Unrealized Capital Losses	18.5		49.5
Embedded Derivatives	20.9	[1]	19.8	[1]
Fixed maturities, including securities pledged, Fair Value	492.5		550.4
OTTI	19.0	[2]	28.8	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	786.8		866.1
Fixed maturities, Gross Unrealized Capital Gains	84.6		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	871.2		911.3
OTTI	4.4	[2]	4.4	[2]
Gross Unrealized Capital Losses	0.2		5.8
Fair Value	871.2		911.3
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	477.7		441.5
Fixed maturities, Gross Unrealized Capital Gains	26.0		19.4
Fixed maturities, Gross Unrealized Capital Losses	10.1		22.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	493.6		438.8
OTTI	3.5	[2]	4.2	[2]
Gross Unrealized Capital Losses	0.6		0.7
Fair Value	436.9		381.0
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	18,626.6		17,662.3
Fixed maturities, Gross Unrealized Capital Gains	2,284.4		1,672.1
Fixed maturities, Gross Unrealized Capital Losses	63.1		153.4
Embedded Derivatives	54.9	[1]	59.2	[1]
Fixed maturities, including securities pledged, Fair Value	20,902.8		19,240.2
OTTI	27.5	[2]	38.1	[2]
Securities pledged, Amortized Cost	201.5		572.5
Securities pledged, Gross Unrealized Capital Gains	13.7		22.4
Securities pledged, Gross Unrealized Capital Losses	1.6		1.2
Securities pledged, Gross Unrealized Capital Losses	213.6		593.7
Total fixed maturities, less securities pledged, Amortized Cost	18,425.1		17,089.8
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	2,270.7		1,649.7
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	61.5		152.2
Total fixed maturities, less securities pledged, Fair Value	20,689.2		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	130.1		131.8
Gross Unrealized Capital Gains	12.4		13.1
Gross Unrealized Capital Losses	0.1		0
Fair Value	$ 142.4		$ 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.