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Supplementary Information - Reserve Quantity Information (Detail)
 12 Months Ended
Dec. 31, 2018
MBbls
MMcf
Dec. 31, 2017
MBbls
MMcf
Oil [Member]    
Proved Developed Reserves:    
Beginning of year 5,333 3,107
Extensions, discoveries and improved recovery 1,389 2,263
Revisions of previous estimates 192 496
Converted from undeveloped reserves 605 383
Reserves sold (3) (2)
Reserve purchased 75 90
Production (1,187) (1,004)
End of year 6,404 5,333
Proved Undeveloped Reserves:    
Beginning of year 505 643
Extensions, discoveries and improved recovery 212 298
Revisions of previous estimates (102) (53)
Converted to developed reserves (605) (383)
End of year 10 505
Total Proved Reserves at the End of the Year 6,414 5,838
NGLs [Member]    
Proved Developed Reserves:    
Beginning of year 1,704 1,265
Extensions, discoveries and improved recovery 511 488
Revisions of previous estimates 662 89
Converted from undeveloped reserves 258 103
Reserve purchased 35 41
Production (463) (282)
End of year 2,707 1,704
Proved Undeveloped Reserves:    
Beginning of year 156 159
Extensions, discoveries and improved recovery 216 118
Revisions of previous estimates (102) (18)
Converted to developed reserves (258) (103)
End of year 12 156
Total Proved Reserves at the End of the Year 2,719 1,860
Gas [Member]    
Proved Developed Reserves:    
Beginning of year | MMcf 17,143 13,001
Extensions, discoveries and improved recovery | MMcf 2,832 3,253
Revisions of previous estimates | MMcf 4,144 3,846
Converted from undeveloped reserves | MMcf 870 476
Reserves sold | MMcf (368) (13)
Reserve purchased | MMcf 179 220
Production | MMcf (3,735) (3,640)
End of year | MMcf 21,065 17,143
Proved Undeveloped Reserves:    
Beginning of year | MMcf 709 2,003
Extensions, discoveries and improved recovery | MMcf 446 335
Revisions of previous estimates | MMcf (161) (1,153)
Converted to developed reserves | MMcf (870) (476)
End of year | MMcf 124 709
Total Proved Reserves at the End of the Year | MMcf 21,189 17,852