Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Reserve Quantities [Line Items]
Future cash inflows	$ 24,851,589	$ 23,954,048
Future costs:
Production	(10,028,359)	(5,113,902)
Development	(3,667,672)	(3,230,577)
Future net cash flows before income taxes	11,155,558	15,609,569
Future income tax expense	(3,081,918)	(5,040,104)
Total future net cash flows before 10% discount	8,073,640	10,569,465
10% annual discount	(4,849,835)	(6,054,568)
Standardized measure of discounted future net cash flows	$ 3,223,805	$ 4,514,897