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Available-for-Sale Securities (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]    
Fair Value $ 4,647.0 $ 4,597.9
Equity Securities | Common Stock
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 871.0 [1] 1,500.7 [1]
Equity Securities | Energy Sector Business | Common Stock
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 573.3 1,004.8
Debt Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 2,679.5 2,832.4
Debt Securities | Energy Sector Business | Residential Mortgage Backed Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 497.3 499.9
Debt Securities | Energy Sector Business | Commercial Mortgage Backed Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 144.7 173.4
Debt Securities | Energy Sector Business | Other Asset Backed Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value $ 218.5 $ 193.2
[1] Of the $871.0 million of fair value as of December 31, 2011, $573.3 million related to certain energy sector businesses. Of the $1,500.7 million of fair value as of December 31, 2010, $1,004.8 million related to certain energy sector businesses.