Investments (Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities, Cost and Fair Value of Equity Securities) (Details) (USD $) In Millions	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Fixed maturities, Amortized Cost or Cost	$ 3,416.7
Fixed maturities, amortized cost	4,466.3	4,598.8
Fixed maturities, Gross Unrealized Gains	275.3	264.7
Fixed maturities, Gross Unrealized Losses	43.7	65.6
Fixed maturities, Fair Value	4,697.9	4,797.9
Total fixed maturities, OTTI Unrealized Losses	22.8	27.8
Equity securities, Amortized Cost or Cost	147.8	120.7
Equity securities, Gross Unrealized Gains	17.0	9.8
Equity securities, Gross Unrealized Losses	0.6	1.9
Equity securities, Fair Value	164.2	128.6
Equity securities, OTTI Unrealized Losses
Net unrealized gains impaired securities	32.3	36.1
U.S. Treasury and Government Agencies [Member]
Fixed maturities, Amortized Cost or Cost	212.2	259.4
Fixed maturities, Gross Unrealized Gains	4.3	5.0
Fixed maturities, Gross Unrealized Losses	2.1	3.2
Fixed maturities, Fair Value	214.4	261.2
Fixed maturities, OTTI Unrealized Losses
Corporate [Member]
Fixed maturities, Amortized Cost or Cost	2,260.3	2,276.0
Fixed maturities, Gross Unrealized Gains	179.5	174.6
Fixed maturities, Gross Unrealized Losses	22.7	30.2
Fixed maturities, Fair Value	2,417.1	2,420.4
Fixed maturities, OTTI Unrealized Losses	16.1	19.5
Municipal [Member]
Fixed maturities, Amortized Cost or Cost	944.2	952.7
Fixed maturities, Gross Unrealized Gains	34.1	21.3
Fixed maturities, Gross Unrealized Losses	9.2	19.3
Fixed maturities, Fair Value	969.1	954.7
Fixed maturities, OTTI Unrealized Losses
Residential Mortgage-Backed [Member]
Fixed maturities, Amortized Cost or Cost	669.5	704.2
Fixed maturities, Gross Unrealized Gains	39.3	41.8
Fixed maturities, Gross Unrealized Losses	9.2	11.9
Fixed maturities, Fair Value	699.6	734.1
Fixed maturities, OTTI Unrealized Losses	6.7	8.3
Commercial Mortgage-Backed [Member]
Fixed maturities, Amortized Cost or Cost	339.9	349.3
Fixed maturities, Gross Unrealized Gains	14.7	18.3
Fixed maturities, Gross Unrealized Losses	0.5	1.0
Fixed maturities, Fair Value	354.1	366.6
Fixed maturities, OTTI Unrealized Losses
Asset-Backed [Member]
Fixed maturities, Amortized Cost or Cost	40.2	57.2
Fixed maturities, Gross Unrealized Gains	3.4	3.7
Fixed maturities, Gross Unrealized Losses
Fixed maturities, Fair Value	43.6	60.9
Fixed maturities, OTTI Unrealized Losses

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