Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Proved Undeveloped Reserves (Detail)	12 Months Ended
	Dec. 31, 2017 MMcfe
Extractive Industries [Abstract]
Beginning proved undeveloped reserves at December 31, 2016	5,302,414
Undeveloped reserves transferred to developed	(1,861,994)
Revisions	308,929	[1]
Purchases/(sales)	(8,907)
Extension and discoveries	3,173,845
Ending proved undeveloped reserves at December 31, 2017	6,914,287

[1] Includes 668 Bcfe of proved undeveloped reserves dropped due to the five year rule which can be included in our future proved reserves as these locations are added back to our five-year development plan.