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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
Sep. 30, 2012
Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]    
Equity securities, Amortized Cost or Cost $ 363.0 $ 239.9
Equity securities, Fair Value 392.8 246.4
Equity securities, OTTI Unrealized Losses     
Equity securities, OTTI Unrealized Losses     
Fixed maturities, Amortized Cost or Cost 6,363.6 6,008.7
Available for sale, Gross Unrealized Gains 454.8 331.0
Available for sale, Gross Unrealized Losses 23.5 55.0
Fixed maturities, Fair Value 6,794.9 6,284.7
Fixed maturities, OTTI Unrealized Losses 12.2 19.9
Equity Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Available for sale, Gross Unrealized Gains 34.8 15.3
Available for sale, Gross Unrealized Losses 5.0 8.8
U.S. Treasury And Government Agencies
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 214.3 261.7
Available for sale, Gross Unrealized Gains 9.4 7.8
Available for sale, Gross Unrealized Losses 0.2 0.2
Fixed maturities, Fair Value 223.5 269.3
Foreign Governments
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 360.3 239.1
Available for sale, Gross Unrealized Gains 4.8 0.4
Available for sale, Gross Unrealized Losses 0.3 0.5
Fixed maturities, Fair Value 364.8 239.0
Municipal
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 951.0 964.5
Available for sale, Gross Unrealized Gains 89.3 67.4
Available for sale, Gross Unrealized Losses 1.7 3.9
Fixed maturities, Fair Value 1,038.6 1,028.0
Corporate
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 3,510.1 3,218.2
Available for sale, Gross Unrealized Gains 279.3 197.7
Available for sale, Gross Unrealized Losses 16.2 40.3
Fixed maturities, Fair Value 3,773.2 3,375.6
Fixed maturities, OTTI Unrealized Losses 9.3 13.8
Residential Mortgage-Backed
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 792.5 816.1
Available for sale, Gross Unrealized Gains 44.5 40.9
Available for sale, Gross Unrealized Losses 4.6 8.4
Fixed maturities, Fair Value 832.4 848.6
Fixed maturities, OTTI Unrealized Losses 2.9 6.1
Commercial Mortgage-Backed
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 342.7 384.1
Available for sale, Gross Unrealized Gains 23.3 15.0
Available for sale, Gross Unrealized Losses 0.5 1.0
Fixed maturities, Fair Value 365.5 398.1
Asset-Backed
   
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturities, Amortized Cost or Cost 192.7 125.0
Available for sale, Gross Unrealized Gains 4.2 1.8
Available for sale, Gross Unrealized Losses   0.7
Fixed maturities, Fair Value $ 196.9 $ 126.1