Summary of Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Extractive Industries [Abstract]
Future cash inflows	$ 7,040,928	$ 6,295,455	$ 6,171,279
Future production costs	(2,062,657)	(1,946,426)	(1,747,806)
Future development costs	(1,431,101)	(1,241,531)	(1,219,214)
Future income taxes	(884,637)	(799,007)	(852,364)
Future net cash flows	2,662,533	2,308,491	2,351,895
10% annual discount	(977,531)	(794,632)	(808,933)
Standardized measure of discounted future net cash flows	1,685,002	1,513,859	1,542,962
Changes in standardized measure of future net cash flows related to estimated proved oil and gas reserves
Standardized measure at beginning of year	1,513,859	1,542,962	957,629
Sales and transfers of oil, gas and NGLs produced, net of production costs	(708,017)	(697,741)	(670,347)
Changes in price, net of future production costs	229,425	(380,841)	502,324
Extensions and discoveries, net of future production and development costs	155,592	178,272	293,168
Changes in estimated future development costs, net of development costs incurred during the period	28,684	212,329	97,852
Revisions of quantity estimates	281,558	76,450	(27,854)
Accretion of discount	202,087	207,292	118,722
Net change in income taxes	(28,084)	22,947	(300,363)
Purchases of reserves in-place		276,389	567,286
Sales of reserves in-place	15,531	2,480	(36,278)
Changes in production rates due to timing and other	(5,633)	73,320	40,823
Net increase (decrease) in standardized measure	171,143	(29,103)	585,333
Standardized measure at end of year	$ 1,685,002	$ 1,513,859	$ 1,542,962