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, 2013	Dec. 31, 2012
Reserve Quantities [Line Items]
Future cash inflows	$ 35,143,097	$ 24,851,589
Future costs:
Production	(10,176,140)	(10,028,359)
Development	(3,938,296)	(3,667,672)
Future net cash flows before income taxes	21,028,661	11,155,558
Future income tax expense	(6,913,196)	(3,081,918)
Total future net cash flows before 10% discount	14,115,465	8,073,640
10% annual discount	(8,253,234)	(4,849,835)
Standardized measure of discounted future net cash flows	$ 5,862,231	$ 3,223,805