Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities and Cost and Fair Value of Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized Cost or Cost	$ 391.2	$ 299.0
Equity securities, Fair Value	435.0	315.8
Equity securities, OTTI Unrealized Losses
Equity securities, OTTI Unrealized Losses
Fixed maturities, Amortized Cost or Cost	6,310.6	6,529.5
Available for sale, Gross Unrealized Gains	432.2	442.7
Available for sale, Gross Unrealized Losses	19.3	20.0
Fixed maturities, Fair Value	6,723.5	6,952.2
Fixed maturities, OTTI Unrealized Losses	11.2	11.0
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Gross Unrealized Gains	46.0	21.6
Available for sale, Gross Unrealized Losses	2.2	4.8
U.S. Treasury and government agencies
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	314.0	317.2
Available for sale, Gross Unrealized Gains	8.1	8.8
Available for sale, Gross Unrealized Losses	0.6	0.4
Fixed maturities, Fair Value	321.5	325.6
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	299.7	348.5
Available for sale, Gross Unrealized Gains	6.0	4.6
Available for sale, Gross Unrealized Losses	0.1	0.2
Fixed maturities, Fair Value	305.6	352.9
Municipal
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	988.2	1,010.2
Available for sale, Gross Unrealized Gains	89.3	87.2
Available for sale, Gross Unrealized Losses	1.4	1.1
Fixed maturities, Fair Value	1,076.1	1,096.3
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	3,443.3	3,512.8
Available for sale, Gross Unrealized Gains	268.8	275.4
Available for sale, Gross Unrealized Losses	13.3	14.8
Fixed maturities, Fair Value	3,698.8	3,773.4
Fixed maturities, OTTI Unrealized Losses	9.2	9.3
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	720.3	769.0
Available for sale, Gross Unrealized Gains	37.3	39.4
Available for sale, Gross Unrealized Losses	3.3	3.2
Fixed maturities, Fair Value	754.3	805.2
Fixed maturities, OTTI Unrealized Losses	2.0	1.7
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	340.6	373.3
Available for sale, Gross Unrealized Gains	18.8	23.2
Available for sale, Gross Unrealized Losses	0.6	0.3
Fixed maturities, Fair Value	358.8	396.2
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	204.5	198.5
Available for sale, Gross Unrealized Gains	3.9	4.1
Fixed maturities, Fair Value	$ 208.4	$ 202.6

Available for Sale Equity Securities Gross Unrealized Other Than Temporary Impairment Losses

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