Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 18,076.2		$ 4,647.0
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,504.6		871.0
Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	1,504.6	[1]	871.0	[1]
Equity Securities | Energy Sector Business | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 572.8		$ 573.3

[1] Of the $1,504.6 million and $871.0 million of fair value as of September 30, 2012 and December 31, 2011, respectively, $572.8 million and $573.3 million, respectively, related to certain energy sector businesses.