Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Proved Undeveloped Reserves for 2013 (Mmcfe) (Detail)	12 Months Ended
	Dec. 31, 2014 MMcfe
Extractive Industries [Abstract]
Beginning proved undeveloped reserves at December 31, 2013	4,009,608
Undeveloped reserves transferred to developed	(620,167)
Revisions	(147,019)	[1]
Purchases/ (sales)	(58,045)
Extension and discoveries	1,776,091
Ending proved undeveloped reserves at December 31, 2014	4,960,468

[1] Includes 611,341 Mmcfe of proved undeveloped reserves dropped due to the five year rule.