Oil and Gas Reserves Information (Unaudited) - Summary of Changes in Net Quantities of Crude Oil and Natural Gas Reserves (Detail)	12 Months Ended
	Dec. 31, 2017 MBbls MMcf	Dec. 31, 2016 MBbls MMcf	Dec. 31, 2015 MBbls MMcf	Dec. 31, 2014 MBbls MMcf
Oil (MBbls)
Proved Reserves:
Beginning of year | MBbls	7,277	9,229	20,854
Revisions of previous estimates | MBbls	1,232	(406)	(5,096)
Extensions and discoveries | MBbls	1	64	231
Sales of minerals in place | MBbls	(7)	(222)	(3,671)
Production | MBbls	(951)	(1,388)	(3,089)
End of year | MBbls	7,552	7,277	9,229
Proved Developed Reserves:
Proved Developed Reserves | MBbls	7,552	7,277	9,229	16,247
Proved Undeveloped Reserves:
Proved Undeveloped Reserves | MBbls				4,607
Oil (MBbls) | Assets Held for Sale
Proved Reserves:
Beginning of year | MBbls	6,950	8,701	16,282
End of year | MBbls	7,116	6,950	8,701
Proved Developed Reserves:
Proved Developed Reserves | MBbls	7,116	6,950	8,701	14,028
Proved Undeveloped Reserves:
Proved Undeveloped Reserves | MBbls				2,253
Natural Gas (MMcf)
Proved Reserves:
Beginning of year | MMcf	872,468	569,596	495,266
Revisions of previous estimates | MMcf	33,721	130,416	(41,437)
Extensions and discoveries | MMcf	291,881	285,076	168,539
Sales of minerals in place | MMcf	(7,593)	(58,942)	(5,096)
Production | MMcf	(73,521)	(53,678)	(47,676)
End of year | MMcf	1,116,956	872,468	569,596
Proved Developed Reserves:
Proved Developed Reserves | MMcf	436,114	321,527	311,130	324,598
Proved Undeveloped Reserves:
Proved Undeveloped Reserves | MMcf	680,842	550,941	258,466	170,668
Natural Gas (MMcf) | Assets Held for Sale
Proved Reserves:
Beginning of year | MMcf	9,915	9,119	14,716
End of year | MMcf	10,484	9,915	9,119
Proved Developed Reserves:
Proved Developed Reserves | MMcf	10,484	9,915	9,119	14,071
Proved Undeveloped Reserves:
Proved Undeveloped Reserves | MMcf				645