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Supplementary Information - Reserve Quantity Information (Detail)
 12 Months Ended
Dec. 31, 2020
MBbls
MMcf
Dec. 31, 2019
MBbls
MMcf
Oil [Member]    
Proved Developed Reserves:    
Beginning of year 4,381 6,404
Extensions, discoveries and improved recovery 11 471
Revisions of previous estimates (995) (1,260)
Converted from undeveloped reserves 25 7
Reserves sold (29) (5)
Reserve purchased 24 6
Production (733) (1,242)
End of year 2,684 4,381
Proved Undeveloped Reserves:    
Beginning of year 1,833 10
Extensions, discoveries and improved recovery   1,834
Revisions of previous estimates (24) (4)
Converted to developed reserves (25) (7)
End of year 1,784 1,833
Total Proved Reserves at the End of the Year 4,468 6,214
NGLs [Member]    
Proved Developed Reserves:    
Beginning of year 2,914 2,707
Extensions, discoveries and improved recovery 7 264
Revisions of previous estimates (239) 506
Converted from undeveloped reserves 5 7
Reserves sold 0 0
Reserve purchased 8 4
Production (437) (574)
End of year 2,258 2,914
Proved Undeveloped Reserves:    
Beginning of year 1,017 12
Extensions, discoveries and improved recovery   1,013
Revisions of previous estimates (224) (1)
Converted to developed reserves (5) (7)
End of year 787 1,017
Total Proved Reserves at the End of the Year 3,045 3,931
Gas [Member]    
Proved Developed Reserves:    
Beginning of year | MMcf 19,995 21,065
Extensions, discoveries and improved recovery | MMcf 36 1,330
Revisions of previous estimates | MMcf (1,721) 2,033
Converted from undeveloped reserves | MMcf 66 65
Reserves sold | MMcf (1,400) (120)
Reserve purchased | MMcf 38 19
Production | MMcf (3,381) (4,397)
End of year | MMcf 13,633 19,995
Proved Undeveloped Reserves:    
Beginning of year | MMcf 4,457 124
Extensions, discoveries and improved recovery | MMcf   4,530
Revisions of previous estimates | MMcf (584) (42)
Converted to developed reserves | MMcf (66) (65)
End of year | MMcf 3,897 4,457
Total Proved Reserves at the End of the Year | MMcf 17,530 24,542