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	$ 28.1	[1]	$ 29.0	[1]
Equity securities, available-for-sale	122.3		134.9
OTTI	16.5	[2]	24.8	[2]
Securities pledged, Amortized Cost	140.9		137.9
Equity securities, cost	106.9		119.4
Total fixed maturities and equity securities, Amortized Cost	19,888.1		19,837.4
Gross Unrealized Capital Gains	1,529.9		1,199.6
Gross Unrealized Capital Losses	104.1		365.4
Fair Value	21,342.0		20,700.6
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	647.9		636.5
Fixed maturities, Gross Unrealized Capital Gains	85.6		36.5
Fixed maturities, Gross Unrealized Capital Losses	0.1		2.9
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	733.4		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	54.8		237.1
Fixed maturities, Gross Unrealized Capital Gains	1.3		5.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	56.1		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	152.7		77.2
Fixed maturities, Gross Unrealized Capital Gains	11.4		5.9
Fixed maturities, Gross Unrealized Capital Losses	0.5		0.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	163.6		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	10,311.5		10,326.0
Fixed maturities, Gross Unrealized Capital Gains	798.9		581.0
Fixed maturities, Gross Unrealized Capital Losses	61.8		238.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	11,048.6		10,668.2
OTTI	1.6	[2]	1.9	[2]
Foreign
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	5,683.2	[3]	5,572.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	376.9	[3]	298.1	[3]
Fixed maturities, Gross Unrealized Capital Losses	35.4	[3]	100.0	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	6,024.7	[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	347.8	[3]	422.9	[3]
Fixed maturities, Gross Unrealized Capital Gains	24.5	[3]	25.2	[3]
Fixed maturities, Gross Unrealized Capital Losses	4.8	[3]	16.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	367.5	[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	5,335.4	[3]	5,149.6	[3]
Fixed maturities, Gross Unrealized Capital Gains	352.4	[3]	272.9	[3]
Fixed maturities, Gross Unrealized Capital Losses	30.6	[3]	83.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,657.2	[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	1,827.7		1,916.3
Fixed maturities, Gross Unrealized Capital Gains	173.2		177.1
Fixed maturities, Gross Unrealized Capital Losses	7.0		22.7
Embedded Derivatives	28.1	[1]	29.0	[1]
Fixed maturities, including securities pledged, Fair Value	2,022.0		2,099.7
OTTI	11.9	[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	1,589.5		1,638.2
Fixed maturities, Gross Unrealized Capital Gains	120.3		121.9
Fixed maturities, Gross Unrealized Capital Losses	5.0		17.9
Embedded Derivatives	16.3	[1]	16.9	[1]
Fixed maturities, including securities pledged, Fair Value	1,721.1		1,759.1
OTTI	0.2	[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	238.2		278.1
Fixed maturities, Gross Unrealized Capital Gains	52.9		55.2
Fixed maturities, Gross Unrealized Capital Losses	2.0		4.8
Embedded Derivatives	11.8	[1]	12.1	[1]
Fixed maturities, including securities pledged, Fair Value	300.9		340.6
OTTI	11.7	[2]	15.1	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	842.6		624.5
Fixed maturities, Gross Unrealized Capital Gains	63.1		68.1
Fixed maturities, Gross Unrealized Capital Losses	0.4		0.9
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	905.3		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	414.0
Fixed maturities, including securities pledged, Amortized Cost	401.7		465.8
Fixed maturities, Gross Unrealized Capital Gains	14.1		18.0
Fixed maturities, Gross Unrealized Capital Losses	1.8		3.4
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	414.0		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	19,922.1		19,855.9
Fixed maturities, Gross Unrealized Capital Gains	1,524.5		1,189.7
Fixed maturities, Gross Unrealized Capital Losses	107.0		368.8
Embedded Derivatives	28.1	[1]	29.0	[1]
Fixed maturities, including securities pledged, Fair Value	21,367.7		20,705.8
OTTI	16.5	[2]	24.8	[2]
Securities pledged, Amortized Cost	140.9		137.9
Securities pledged, Gross Unrealized Capital Gains	10.0		5.9
Securities pledged, Gross Unrealized Capital Losses	2.9		3.7
Securities pledged, Gross Unrealized Capital Losses	148.0		140.1
Total fixed maturities, less securities pledged, Amortized Cost	19,781.2		19,718.0
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,514.5		1,183.8
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	104.1		365.1
Total fixed maturities, less securities pledged, Fair Value	21,219.7		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	122.3		134.9
OTTI	0	[2]	0	[2]
Equity securities, cost	106.9		119.4
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.4		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.