Investments Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Jun. 30, 2012		Dec. 31, 2011
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives		$ 51.1	[1]	$ 55.0	[1]
OTTI		64.7	[2]	78.0	[2]
Equity securities cost	26.5	26.5		26.7
Total fixed maturities and equity securities, Amortized Cost		19,322.1		20,424.1
Gross Unrealized Capital Gains		2,102.8		1,582.1
Gross Unrealized Capital Losses		127.0		297.8
Fair value		21,349.0		21,763.4
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		1,268.6		1,692.9
Fixed maturities, Gross Unrealized Capital Gain		100.2		92.9
Fixed maturities, Gross Unrealized Capital Losses		0		0
Embedded Derivatives		0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value		1,368.8		1,785.8
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		19.3		19.9
Fixed maturities, Gross Unrealized Capital Gain		4.5		3.8
Fixed maturities, Gross Unrealized Capital Losses		0		0
Embedded Derivatives		0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value		23.8		23.7
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		80.1		98.9
Fixed maturities, Gross Unrealized Capital Gain		9.3		6.8
Fixed maturities, Gross Unrealized Capital Losses		0.4		0.9
Embedded Derivatives		0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value		89.0		104.8
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		9,586.6		9,527.7
Fixed maturities, Gross Unrealized Capital Gain		1,060.5		784.5
Fixed maturities, Gross Unrealized Capital Losses		22.9		41.4
Embedded Derivatives		0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value		10,624.2		10,270.8
OTTI		0	[2]	0	[2]
Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		4,873.3	[3]	5,288.4	[3]
Fixed maturities, Gross Unrealized Capital Gain		502.3	[3]	363.5	[3]
Fixed maturities, Gross Unrealized Capital Losses		27.9	[3]	69.8	[3]
Embedded Derivatives		0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value		5,347.7	[3]	5,582.1	[3]
OTTI		0	[2],[3]	0.1	[2],[3]
Foreign government
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		415.3	[3]	349.0	[3]
Fixed maturities, Gross Unrealized Capital Gain		37.4	[3]	26.7	[3]
Fixed maturities, Gross Unrealized Capital Losses		2.7	[3]	5.4	[3]
Embedded Derivatives		0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value		450.0	[3]	370.3	[3]
OTTI		0	[2],[3]	0	[2],[3]
Foreign other
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		4,458.0	[3]	4,939.4	[3]
Fixed maturities, Gross Unrealized Capital Gain		464.9	[3]	336.8	[3]
Fixed maturities, Gross Unrealized Capital Losses		25.2	[3]	64.4	[3]
Embedded Derivatives		0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value		4,897.7	[3]	5,211.8	[3]
OTTI		0	[2],[3]	0.1	[2],[3]
Residential mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		1,750.7		2,090.0
Fixed maturities, Gross Unrealized Capital Gain		229.0		245.2
Fixed maturities, Gross Unrealized Capital Losses		36.6		90.9
Embedded Derivatives		55.5	[1]	61.5	[1]
Fixed maturities, including securities pledged, Fair Value		1,998.6		2,305.8
OTTI		64.4	[2]	76.0	[2]
Agency Residential Mortgage-backed Securities [Member]
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		1,166.2		1,354.6
Fixed maturities, Gross Unrealized Capital Gain		158.9		186.9
Fixed maturities, Gross Unrealized Capital Losses		4.4		2.4
Embedded Derivatives		39.5	[1]	46.4	[1]
Fixed maturities, including securities pledged, Fair Value		1,360.2		1,585.5
OTTI		0	[2]	0.3	[2]
Non-agency Residential Mortgage-backed Securities [Member]
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		584.5		735.4
Fixed maturities, Gross Unrealized Capital Gain		70.1		58.3
Fixed maturities, Gross Unrealized Capital Losses		32.2		88.5
Embedded Derivatives		16.0	[1]	15.1	[1]
Fixed maturities, including securities pledged, Fair Value		638.4		720.3
OTTI		64.4	[2]	75.7	[2]
Commercial mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		1,630.2		1,910.3
Fixed maturities, Gross Unrealized Capital Gain		200.7		118.0
Fixed maturities, Gross Unrealized Capital Losses		4.5		26.4
Embedded Derivatives		0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value		1,826.4		2,001.9
OTTI		0	[2]	1.9	[2]
Gross Unrealized Capital Losses	4.5			26.4
Fair value	1,800.0			2,000.0
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		706.1		734.3
Fixed maturities, Gross Unrealized Capital Gain		24.3		15.4
Fixed maturities, Gross Unrealized Capital Losses		36.2		69.6
Embedded Derivatives		(4.4)	[1]	(6.5)	[1]
Fixed maturities, including securities pledged, Fair Value		689.8		673.6
OTTI		0.3	[2]	0	[2]
Gross Unrealized Capital Losses				0.2
Fair value	507.1			489.7
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost		19,914.9		21,362.4
Fixed maturities, Gross Unrealized Capital Gain		2,130.8		1,630.1
Fixed maturities, Gross Unrealized Capital Losses		128.5		299.0
Embedded Derivatives		51.1	[1]	55.0	[1]
Fixed maturities, including securities pledged, Fair Value		21,968.3		22,748.5
OTTI		64.7	[2]	78.0	[2]
Securities pledged, Amortized Cost		619.3		965.0
Securities pledged, Gross Unrealized Capital Gains		31.6		49.8
Securities pledged, Gross Unrealized Capital Losses		1.8		2.0
Securities pledged, Fair Value		649.1		1,012.8
Total fixed maturities, less securities pledged, Amortized Cost		19,295.6		20,397.4
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains		2,099.2		1,580.3
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses		126.7		297.0
Total fixed maturities, less securities pledged, Fair Value		21,319.2		21,735.7
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]
Gross Unrealized Capital Gains		3.6		1.8
Gross Unrealized Capital Losses		0.3		0.8
Fair value		$ 29.8		$ 27.7

[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 Statements of Operations.

[2] Represents other-than-temporary impairments ("OTTI") reported as a component of Other comprehensive income.

[3] Primarily U.S. dollar denominated.