Fair Value of Assets and Liabilities Measurement Using Unobservable Input Or Level 3 (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Natural gas, oil and NGL sales
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total gains (losses) included in earnings for the period	$ 525	$ 256
Change in unrealized gains (losses) relating to assets still held at reporting date	370	133
Interest Expense
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total gains (losses) included in earnings for the period	4	21
Change in unrealized gains (losses) relating to assets still held at reporting date	$ (2)	$ 0