Supplemental Information on Oil and Natural Gas Operations - Unaudited - Summary of Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Extractive Industries [Abstract]
Future cash inflows	$ 6,635,751	$ 7,040,928	$ 6,295,455
Future production costs	(2,413,004)	(2,062,657)	(1,946,426)
Future development costs	(1,511,687)	(1,431,101)	(1,241,531)
Future income taxes	(609,516)	(884,637)	(799,007)
Future net cash flows	2,101,544	2,662,533	2,308,491
10% annual discount	(682,752)	(977,531)	(794,632)
Standardized measure of discounted future net cash flows	1,418,792	1,685,002	1,513,859
Changes in standardized measure of future net cash flows related to estimated proved oil and gas reserves
Standardized measure at beginning of year	1,685,002	1,513,859	1,542,962
Sales and transfers of oil, gas and NGLs produced, net of production costs	(486,232)	(708,017)	(697,741)
Changes in price, net of future production costs	(864,118)	229,425	(380,841)
Extensions and discoveries, net of future production and development costs	549,649	155,592	178,272
Changes in estimated future development costs, net of development costs incurred during the period	203,026	28,684	212,329
Revisions of quantity estimates	(27,495)	281,558	76,450
Accretion of discount	222,009	202,087	207,292
Net change in income taxes	209,323	(28,084)	22,947
Purchases of reserves in-place			276,389
Sales of reserves in-place	(152,787)	15,531	2,480
Changes in production rates due to timing and other	80,415	(5,633)	73,320
Net increase (decrease) in standardized measure	(266,210)	171,143	(29,103)
Standardized measure at end of year	$ 1,418,792	$ 1,685,002	$ 1,513,859