Investments (Amortized Cost And Fair Value Of Available-For-Sale Fixed Maturities, Cost And Fair Value Of Equity Securities) (Details) (USD $) In Millions	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	$ 5,966.1	$ 4,598.8	$ 4,598.8
Fixed maturities, Gross Unrealized Gains	323.5	264.7
Fixed maturities, Gross Unrealized Losses	62.0	65.6
Fixed maturities, Fair Value	6,227.6	4,797.9	4,797.9
Fixed maturities, OTTI Unrealized Losses	22.0	27.8
Equity securities, Amortized Cost or Cost	249.2	120.7	120.7
Equity securities, Gross Unrealized Gains	9.7	9.8
Equity securities, Gross Unrealized Losses	12.6	1.9
Equity securities, Fair Value	246.3	128.6	128.6
U.S. Treasury And Government Agencies [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	272.0	259.4
Fixed maturities, Gross Unrealized Gains	7.3	5.0
Fixed maturities, Gross Unrealized Losses	0.3	3.2
Fixed maturities, Fair Value	279.0	261.2
Foreign Governments [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	302.6
Fixed maturities, Gross Unrealized Gains	0.4
Fixed maturities, Gross Unrealized Losses	0.1
Fixed maturities, Fair Value	302.9
Corporate [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	3,150.7	2,276.0
Fixed maturities, Gross Unrealized Gains	188.3	174.6
Fixed maturities, Gross Unrealized Losses	47.6	30.2
Fixed maturities, Fair Value	3,291.4	2,420.4
Fixed maturities, OTTI Unrealized Losses	15.7	19.5
Municipal [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	970.0	952.7
Fixed maturities, Gross Unrealized Gains	67.2	21.3
Fixed maturities, Gross Unrealized Losses	4.3	19.3
Fixed maturities, Fair Value	1,032.9	954.7
Residential Mortgage-Backed [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	825.7	704.2
Fixed maturities, Gross Unrealized Gains	47.0	41.8
Fixed maturities, Gross Unrealized Losses	8.0	11.9
Fixed maturities, Fair Value	864.7	734.1
Fixed maturities, OTTI Unrealized Losses	6.3	8.3
Commercial Mortgage-Backed [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	338.1	349.3
Fixed maturities, Gross Unrealized Gains	9.3	18.3
Fixed maturities, Gross Unrealized Losses	1.3	1.0
Fixed maturities, Fair Value	346.1	366.6
Asset-Backed [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturities, Amortized Cost or Cost	107.0	57.2
Fixed maturities, Gross Unrealized Gains	4.0	3.7
Fixed maturities, Gross Unrealized Losses	0.4
Fixed maturities, Fair Value	$ 110.6	$ 60.9

Available For Sale Securities OTTI Unrealized Loss

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Equity Securities, Excluding Discontinued Operations, Gross Unrealized Gains

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