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	$ 28.5	[1]	$ 29.0	[1]
Equity securities, available-for-sale	127.0		134.9
OTTI	18.2	[2]	24.8	[2]
Securities pledged, Amortized Cost	204.6		137.9
Equity securities, cost	111.1		119.4
Total fixed maturities and equity securities, Amortized Cost	19,810.6		19,837.4
Gross Unrealized Capital Gains	1,465.2		1,199.6
Gross Unrealized Capital Losses	179.2		365.4
Fair Value	21,125.1		20,700.6
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	622.4		636.5
Fixed maturities, Gross Unrealized Capital Gains	60.6		36.5
Fixed maturities, Gross Unrealized Capital Losses	1.9		2.9
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	681.1		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	237.0		237.1
Fixed maturities, Gross Unrealized Capital Gains	2.7		5.0
Fixed maturities, Gross Unrealized Capital Losses	0		0
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	239.7		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	77.2		77.2
Fixed maturities, Gross Unrealized Capital Gains	9.3		5.9
Fixed maturities, Gross Unrealized Capital Losses	0		0.1
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	86.5		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	10,381.3		10,326.0
Fixed maturities, Gross Unrealized Capital Gains	756.9		581.0
Fixed maturities, Gross Unrealized Capital Losses	107.3		238.8
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	11,030.9		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	5,580.9	[3]	5,572.5	[3]
Fixed maturities, Gross Unrealized Capital Gains	364.2	[3]	298.1	[3]
Fixed maturities, Gross Unrealized Capital Losses	55.1	[3]	100.0	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,890.0	[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	363.0	[3]	422.9	[3]
Fixed maturities, Gross Unrealized Capital Gains	23.6	[3]	25.2	[3]
Fixed maturities, Gross Unrealized Capital Losses	9.4	[3]	16.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	377.2	[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	5,217.9	[3]	5,149.6	[3]
Fixed maturities, Gross Unrealized Capital Gains	340.6	[3]	272.9	[3]
Fixed maturities, Gross Unrealized Capital Losses	45.7	[3]	83.5	[3]
Embedded Derivatives	0	[1],[3]	0	[1],[3]
Fixed maturities, including securities pledged, Fair Value	5,512.8	[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	1,901.5		1,916.3
Fixed maturities, Gross Unrealized Capital Gains	180.8		177.1
Fixed maturities, Gross Unrealized Capital Losses	14.5		22.7
Embedded Derivatives	28.5	[1]	29.0	[1]
Fixed maturities, including securities pledged, Fair Value	2,096.3		2,099.7
OTTI	13.3	[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	1,635.9		1,638.2
Fixed maturities, Gross Unrealized Capital Gains	123.3		121.9
Fixed maturities, Gross Unrealized Capital Losses	11.9		17.9
Embedded Derivatives	16.3	[1]	16.9	[1]
Fixed maturities, including securities pledged, Fair Value	1,763.6		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	265.6		278.1
Fixed maturities, Gross Unrealized Capital Gains	57.5		55.2
Fixed maturities, Gross Unrealized Capital Losses	2.6		4.8
Embedded Derivatives	12.2	[1]	12.1	[1]
Fixed maturities, including securities pledged, Fair Value	332.7		340.6
OTTI	13.1	[2]	15.1	[2]
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	655.4		624.5
Fixed maturities, Gross Unrealized Capital Gains	68.2		68.1
Fixed maturities, Gross Unrealized Capital Losses	0.1		0.9
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	723.5		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	461.9
Fixed maturities, including securities pledged, Amortized Cost	448.4		465.8
Fixed maturities, Gross Unrealized Capital Gains	16.4		18.0
Fixed maturities, Gross Unrealized Capital Losses	2.9		3.4
Embedded Derivatives	0	[1]	0	[1]
Fixed maturities, including securities pledged, Fair Value	461.9		480.4
OTTI	3.1	[2]	3.2	[2]
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	19,904.1		19,855.9
Fixed maturities, Gross Unrealized Capital Gains	1,459.1		1,189.7
Fixed maturities, Gross Unrealized Capital Losses	181.8		368.8
Embedded Derivatives	28.5	[1]	29.0	[1]
Fixed maturities, including securities pledged, Fair Value	21,209.9		20,705.8
OTTI	18.2	[2]	24.8	[2]
Securities pledged, Amortized Cost	204.6		137.9
Securities pledged, Gross Unrealized Capital Gains	9.8		5.9
Securities pledged, Gross Unrealized Capital Losses	2.6		3.7
Securities pledged, Gross Unrealized Capital Losses	211.8		140.1
Total fixed maturities, less securities pledged, Amortized Cost	19,699.5		19,718.0
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	1,449.3		1,183.8
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	179.2		365.1
Total fixed maturities, less securities pledged, Fair Value	20,998.1		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	127.0		134.9
OTTI	0	[2]	0	[2]
Equity securities, cost	111.1		119.4
Available-for-sale Equity Securities, Gross Unrealized Gain Accumulated In Investments	15.9		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.

[3] Primarily U.S. dollar denominated.