Investments - Fixed Maturities and Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	$ 27.7	[1]	$ 29.0	[1]
Equity securities, available-for-sale	123.1		134.9
OTTI	17.5	[2]	24.8	[2]
Securities pledged, Amortized Cost	223.1		137.9
Equity securities, cost	107.5		119.4
Total fixed maturities and equity securities, Amortized Cost	19,738.3		19,837.4
Gross Unrealized Capital Gains	1,726.3		1,199.6
Gross Unrealized Capital Losses	78.1		365.4
Fair Value	21,414.2		20,700.6
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	692.5		636.5
Fixed maturities, Gross Unrealized Capital Gains	78.9		36.5
Fixed maturities, Gross Unrealized Capital Losses	0.2		2.9
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	771.2		670.1
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	54.9		237.1
Fixed maturities, Gross Unrealized Capital Gains	1.9		5.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	56.8		242.1
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	109.1		77.2
Fixed maturities, Gross Unrealized Capital Gains	11.6		5.9
Fixed maturities, Gross Unrealized Capital Losses	0		0.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	120.7		83.0
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	10,304.5		10,326.0
Fixed maturities, Gross Unrealized Capital Gains	921.9		581.0
Fixed maturities, Gross Unrealized Capital Losses	45.9		238.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	11,180.5		10,668.2
OTTI	1.8	[2]	1.9	[2]
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,583.0	[3]	5,572.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	437.1	[3]	298.1	[3]
Fixed maturities, Gross Unrealized Capital Losses	23.0	[3]	100.0	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,997.1	[3]	5,770.6	[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	362.3	[3]	422.9	[3]
Fixed maturities, Gross Unrealized Capital Gains	28.3	[3]	25.2	[3]
Fixed maturities, Gross Unrealized Capital Losses	4.5	[3]	16.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	386.1	[3]	431.6	[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	5,220.7	[3]	5,149.6	[3]
Fixed maturities, Gross Unrealized Capital Gains	408.8	[3]	272.9	[3]
Fixed maturities, Gross Unrealized Capital Losses	18.5	[3]	83.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,611.0	[3]	5,339.0	[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	1,891.8		1,916.3
Fixed maturities, Gross Unrealized Capital Gains	188.0		177.1
Fixed maturities, Gross Unrealized Capital Losses	8.8		22.7
Embedded Derivatives	27.7	[1]	29.0	[1]
Fixed maturities, including securities pledged, Fair Value	2,098.7		2,099.7
OTTI	12.7	[2]	15.3	[2]
Residential mortgage-backed securities agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,638.3		1,638.2
Fixed maturities, Gross Unrealized Capital Gains	130.7		121.9
Fixed maturities, Gross Unrealized Capital Losses	6.8		17.9
Embedded Derivatives	15.5	[1]	16.9	[1]
Fixed maturities, including securities pledged, Fair Value	1,777.7		1,759.1
OTTI	0.1	[2]	0.2	[2]
Residential mortgage-backed securities non-agency
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	253.5		278.1
Fixed maturities, Gross Unrealized Capital Gains	57.3		55.2
Fixed maturities, Gross Unrealized Capital Losses	2.0		4.8
Embedded Derivatives	12.2	[1]	12.1	[1]
Fixed maturities, including securities pledged, Fair Value	321.0		340.6
OTTI	12.6	[2]	15.1	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	756.4		624.5
Fixed maturities, Gross Unrealized Capital Gains	70.2		68.1
Fixed maturities, Gross Unrealized Capital Losses	0.2		0.9
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	826.4		691.7
OTTI	0	[2]	4.4	[2]
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Fair Value	476.1
Fixed maturities, including securities pledged, Amortized Cost	461.7		465.8
Fixed maturities, Gross Unrealized Capital Gains	16.4		18.0
Fixed maturities, Gross Unrealized Capital Losses	2.0		3.4
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	476.1		480.4
OTTI	3.0	[2]	3.2	[2]
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	19,853.9		19,855.9
Fixed maturities, Gross Unrealized Capital Gains	1,726.0		1,189.7
Fixed maturities, Gross Unrealized Capital Losses	80.1		368.8
Embedded Derivatives	27.7	[1]	29.0	[1]
Fixed maturities, including securities pledged, Fair Value	21,527.5		20,705.8
OTTI	17.5	[2]	24.8	[2]
Securities pledged, Amortized Cost	223.1		137.9
Securities pledged, Gross Unrealized Capital Gains	15.3		5.9
Securities pledged, Gross Unrealized Capital Losses	2.0		3.7
Securities pledged, Gross Unrealized Capital Losses	236.4		140.1
Total fixed maturities, less securities pledged, Amortized Cost	19,630.8		19,718.0
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,710.7		1,183.8
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	78.1		365.1
Total fixed maturities, less securities pledged, Fair Value	21,291.1		20,565.7
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Embedded Derivatives	0	[1]	0	[1]
Equity securities, available-for-sale	123.1		134.9
OTTI	0	[2]	0	[2]
Equity securities, cost	107.5		119.4
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.6		15.8
Available-for-sale Equity Securities, Gross Unrealized Loss Accumulated In Investments	$ 0		$ 0.3

[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 (loss).

[3] Primarily U.S. dollar denominated.