Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	$ 17,872.5		$ 17,789.5
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,004.9		1,424.0
Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of financial instruments	2,004.9	[1]	1,424.0	[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	$ 775.0		$ 542.2

[1] Of the $2,004.9 million and $1,424.0 million of fair value as of March 31, 2013 and December 31, 2012, respectively, $775.0 million and $542.2 million, respectively, related to certain energy sector businesses.