Supplemental Oil and Gas Disclosures - Unaudited Information of Future Net Cash Flows for Oil and Gas Reserves, Net of Income Tax Expense (Parenthetical) (Detail) (USD $) In Billions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Estimated future net cash flow before income tax expenses	10.00%
Total estimated future net cash flows before income tax expense discounted at 10 percent per annum	$ 48.2	$ 52.3