Investments - Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	$ 118.2	[1]	$ 126.5
OTTI	95.8	[2]	126.0
Securities pledged, Amortized Cost	951.7		1,457.9
Securities pledged, Gross Unrealized Capital Losses	1,021.6		1,465.7
Total equity securities, Amortized Cost	239.2		267.4
Equity securities, available-for-sale	270.5		314.4
Total fixed maturities and equity securities, Amortized Cost	68,662.1		68,236.5
Total fixed maturities and equity securities, Gross Unrealized Capital Gains	5,545.1		4,325.7
Total fixed maturities and equity securities, Gross Unrealized Capital Losses	341.4		1,121.2
Total fixed maturities and equity securities, Fair Value	73,984.0		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	44.00%		38.30%
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	3,767.5		5,094.0
Fixed maturities, Gross Unrealized Capital Gains	424.7		174.0
Fixed maturities, Gross Unrealized Capital Losses	5.1		86.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	4,187.1		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	415.3		598.0
Fixed maturities, Gross Unrealized Capital Gains	47.1		22.3
Fixed maturities, Gross Unrealized Capital Losses	0.1		1.4
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	462.3		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	434.8		272.0
Fixed maturities, Gross Unrealized Capital Gains	21.5		10.6
Fixed maturities, Gross Unrealized Capital Losses	1.5		1.5
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	454.8		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	37,243.2		36,010.3
Fixed maturities, Gross Unrealized Capital Gains	3,054.6		2,174.5
Fixed maturities, Gross Unrealized Capital Losses	219.6		706.2
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	40,078.2		37,478.6
OTTI	10.6	[2]	12.8	[2]
Foreign
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	15,977.9	[3]	15,661.4
Fixed maturities, Gross Unrealized Capital Gains	1,112.2	[3]	913.8
Fixed maturities, Gross Unrealized Capital Losses	72.6	[3]	218.7
Embedded Derivatives	0	[1],[3]	0
Fixed maturities, including securities pledged	17,017.5	[3]	16,356.5
OTTI	0	[2],[3]	0
Foreign securities government
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	862.6	[3]	1,044.0	[3]
Fixed maturities, Gross Unrealized Capital Gains	47.2	[3]	49.6	[3]
Fixed maturities, Gross Unrealized Capital Losses	12.8	[3]	42.2	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged	897.0	[3]	1,051.4
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	15,115.3	[3]	14,617.4	[3]
Fixed maturities, Gross Unrealized Capital Gains	1,065.0	[3]	864.2	[3]
Fixed maturities, Gross Unrealized Capital Losses	59.8	[3]	176.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged	16,120.5	[3]	15,305.1
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	6,024.4		6,480.3
Fixed maturities, Gross Unrealized Capital Gains	582.6		597.3
Fixed maturities, Gross Unrealized Capital Losses	28.8		80.4
Embedded Derivatives	118.2	[1]	126.5
Fixed maturities, including securities pledged	6,696.4		7,123.7
OTTI	80.4	[2]	103.6
Residential mortgage-backed securities, Agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	4,988.3		5,379.2
Fixed maturities, Gross Unrealized Capital Gains	414.0		431.1
Fixed maturities, Gross Unrealized Capital Losses	20.5		62.1
Embedded Derivatives	75.0	[1]	79.2	[1]
Fixed maturities, including securities pledged	5,456.8		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	1,036.1		1,101.1
Fixed maturities, Gross Unrealized Capital Gains	168.6		166.2
Fixed maturities, Gross Unrealized Capital Losses	8.3		18.3
Embedded Derivatives	43.2	[1]	47.3	[1]
Fixed maturities, including securities pledged	1,239.6		1,296.3
OTTI	80.1	[2]	103.2	[2]
Commercial mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	3,761.5		3,427.9
Fixed maturities, Gross Unrealized Capital Gains	270.5		327.7
Fixed maturities, Gross Unrealized Capital Losses	1.2		3.5
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	4,030.8		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	1,750.0		1,883.1
Fixed maturities, Gross Unrealized Capital Gains	78.2		81.6
Fixed maturities, Gross Unrealized Capital Losses	20.2		38.0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged	1,808.0		1,926.7
OTTI	4.8	[2]	5.2	[2]
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	69,374.6		69,427.0
Fixed maturities, Gross Unrealized Capital Gains	5,591.4		4,301.8
Fixed maturities, Gross Unrealized Capital Losses	349.1		1,136.5
Embedded Derivatives	118.2	[1]	126.5
Fixed maturities, including securities pledged	74,735.1		72,718.8
OTTI	95.8	[2]	126.0
Securities pledged, Amortized Cost	951.7		1,457.9
Securities pledged, Gross Unrealized Capital Gains	77.6		24.6
Securities pledged, Gross Unrealized Capital Losses	7.7		16.8
Securities pledged, Gross Unrealized Capital Losses	1,021.6		1,465.7
Total fixed maturities, less securities pledged, Amortized Cost	68,422.9		67,969.1
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	5,513.8		4,277.2
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	341.4		1,119.7
Total fixed maturities, less securities pledged, Fair Value	73,713.5		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	188.7		214.3
Equity securities, Gross Unrealized Capital Gains	0.5		5.1
Equity securities, Gross Unrealized Capital Losses	0		0.9
Equity securities, available-for-sale	189.2		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	30.8		43.4
Equity securities, Gross Unrealized Capital Losses	0		0.6
Equity securities, available-for-sale	81.3		95.9
Equity securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0
OTTI	0	[2]	0
Total equity securities, Amortized Cost	239.2		267.4
Equity securities, Gross Unrealized Capital Gains	31.3		48.5
Equity securities, Gross Unrealized Capital Losses	0		1.5
Equity securities, available-for-sale	$ 270.5		$ 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.