Investments - Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	$ 123.1	[1]	$ 126.5	[1]
OTTI	110.4	[2]	126.0	[2]
Securities pledged, Amortized Cost	1,230.3		1,457.9
Securities pledged, Gross Unrealized Capital Losses	1,271.3		1,465.7
Total equity securities, Amortized Cost	244.8		267.4
Equity securities, available-for-sale	276.6		314.4
Total fixed maturities and equity securities, Amortized Cost	68,934.8		68,236.5
Total fixed maturities and equity securities, Gross Unrealized Capital Gains	5,345.2		4,325.7
Total fixed maturities and equity securities, Gross Unrealized Capital Losses	594.7		1,121.2
Total fixed maturities and equity securities, Fair Value	73,808.4		71,567.5
Mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	41.50%		38.30%
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	4,991.6		5,094.0
Fixed maturities, Gross Unrealized Capital Gains	315.6		174.0
Fixed maturities, Gross Unrealized Capital Losses	44.1		86.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	5,263.1		5,181.2
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	610.6		598.0
Fixed maturities, Gross Unrealized Capital Gains	33.7		22.3
Fixed maturities, Gross Unrealized Capital Losses	0.2		1.4
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	644.1		618.9
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	276.5		272.0
Fixed maturities, Gross Unrealized Capital Gains	17.5		10.6
Fixed maturities, Gross Unrealized Capital Losses	0.4		1.5
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	293.6		281.1
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	36,658.8		36,010.3
Fixed maturities, Gross Unrealized Capital Gains	2,876.3		2,174.5
Fixed maturities, Gross Unrealized Capital Losses	361.6		706.2
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	39,173.5		37,478.6
OTTI	11.8	[2]	12.8	[2]
Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	15,746.2	[3]	15,661.4	[3]
Fixed maturities, Gross Unrealized Capital Gains	1,107.2	[3]	913.8	[3]
Fixed maturities, Gross Unrealized Capital Losses	112.8	[3]	218.7	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged	16,740.6	[3]	16,356.5	[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	886.3	[3]	1,044.0	[3]
Fixed maturities, Gross Unrealized Capital Gains	45.0	[3]	49.6	[3]
Fixed maturities, Gross Unrealized Capital Losses	24.1	[3]	42.2	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged	907.2	[3]	1,051.4	[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	14,859.9	[3]	14,617.4	[3]
Fixed maturities, Gross Unrealized Capital Gains	1,062.2	[3]	864.2	[3]
Fixed maturities, Gross Unrealized Capital Losses	88.7	[3]	176.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged	15,833.4	[3]	15,305.1	[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	6,374.4		6,480.3
Fixed maturities, Gross Unrealized Capital Gains	608.0		597.3
Fixed maturities, Gross Unrealized Capital Losses	52.8		80.4
Embedded Derivatives	123.1	[1]	126.5	[1]
Fixed maturities, including securities pledged	7,052.7		7,123.7
OTTI	93.6	[2]	103.6	[2]
Residential mortgage-backed securities, Agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,319.7		5,379.2
Fixed maturities, Gross Unrealized Capital Gains	432.7		431.1
Fixed maturities, Gross Unrealized Capital Losses	41.9		62.1
Embedded Derivatives	76.2	[1]	79.2	[1]
Fixed maturities, including securities pledged	5,786.7		5,827.4
OTTI	0.3	[2]	0.4	[2]
Residential mortgage-backed securities, Non-Agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,054.7		1,101.1
Fixed maturities, Gross Unrealized Capital Gains	175.3		166.2
Fixed maturities, Gross Unrealized Capital Losses	10.9		18.3
Embedded Derivatives	46.9	[1]	47.3	[1]
Fixed maturities, including securities pledged	1,266.0		1,296.3
OTTI	93.3	[2]	103.2	[2]
Commercial mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	3,472.8		3,427.9
Fixed maturities, Gross Unrealized Capital Gains	323.9		327.7
Fixed maturities, Gross Unrealized Capital Losses	0.6		3.5
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	3,796.1		3,752.1
OTTI	0	[2]	4.4	[2]
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,789.4		1,883.1
Fixed maturities, Gross Unrealized Capital Gains	81.4		81.6
Fixed maturities, Gross Unrealized Capital Losses	31.4		38.0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	1,839.4		1,926.7
OTTI	5.0	[2]	5.2	[2]
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	69,920.3		69,427.0
Fixed maturities, Gross Unrealized Capital Gains	5,363.6		4,301.8
Fixed maturities, Gross Unrealized Capital Losses	603.9		1,136.5
Embedded Derivatives	123.1	[1]	126.5	[1]
Fixed maturities, including securities pledged	74,803.1		72,718.8
OTTI	110.4	[2]	126.0	[2]
Securities pledged, Amortized Cost	1,230.3		1,457.9
Securities pledged, Gross Unrealized Capital Gains	50.4		24.6
Securities pledged, Gross Unrealized Capital Losses	9.4		16.8
Securities pledged, Gross Unrealized Capital Losses	1,271.3		1,465.7
Total fixed maturities, less securities pledged, Amortized Cost	68,690.0		67,969.1
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	5,313.2		4,277.2
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	594.5		1,119.7
Total fixed maturities, less securities pledged, Fair Value	73,531.8		71,253.1
Common Stock
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0	[1]
OTTI	0	[2]	0	[2]
Total equity securities, Amortized Cost	194.3		214.3
Equity securities, Gross Unrealized Capital Gains	2.3		5.1
Equity securities, Gross Unrealized Capital Losses	0.2		0.9
Equity securities, available-for-sale	196.4		218.5
Preferred stock
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0	[1]
OTTI	0	[2]	0	[2]
Total equity securities, Amortized Cost	50.5		53.1
Equity securities, Gross Unrealized Capital Gains	29.7		43.4
Equity securities, Gross Unrealized Capital Losses	0		0.6
Equity securities, available-for-sale	80.2		95.9
Equity securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0	[1]
OTTI	0	[2]	0	[2]
Total equity securities, Amortized Cost	244.8		267.4
Equity securities, Gross Unrealized Capital Gains	32.0		48.5
Equity securities, Gross Unrealized Capital Losses	0.2		1.5
Equity securities, available-for-sale	$ 276.6		$ 314.4

[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 OTTI reported as a component of Other comprehensive income.

[3] Primarily U.S. dollar denominated.