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Supplementary Information - Reserve Quantity Information (Detail)
 12 Months Ended
Dec. 31, 2017
MMcf
MBbls
Dec. 31, 2016
MMcf
MBbls
Oil [Member]    
Proved Developed Reserves:    
Beginning of year 3,107 4,579
Extensions, discoveries and improved recovery 2,263 577
Revisions of previous estimates 496 (1,425)
Converted from undeveloped reserves 383 46
Reserves sold (2)  
Reserve purchased 90  
Production (1,004) (670)
End of year 5,333 3,107
Proved Undeveloped Reserves:    
Beginning of year 643 52
Extensions, discoveries and improved recovery 298 635
Revisions of previous estimates (53) 2
Converted to developed reserves (383) (46)
End of year 505 643
Total Proved Reserves at the End of the Year 5,838 3,750
NGLs [Member]    
Proved Developed Reserves:    
Beginning of year 1,265 1,673
Extensions, discoveries and improved recovery 488 176
Revisions of previous estimates 89 (527)
Converted from undeveloped reserves 103 14
Reserves sold   (1)
Reserve purchased 41  
Production (282) (70)
End of year 1,704 1,265
Proved Undeveloped Reserves:    
Beginning of year 159 12
Extensions, discoveries and improved recovery 118 157
Revisions of previous estimates (18) 4
Converted to developed reserves (103) (14)
End of year 156 159
Total Proved Reserves at the End of the Year 1,860 1,424
Gas [Member]    
Proved Developed Reserves:    
Beginning of year | MMcf 13,001 23,275
Extensions, discoveries and improved recovery | MMcf 3,253 1,136
Revisions of previous estimates | MMcf 3,846 (7,342)
Converted from undeveloped reserves | MMcf 476 65
Reserves sold | MMcf (13) (7)
Reserve purchased | MMcf 220  
Production | MMcf (3,640) (4,126)
End of year | MMcf 17,143 13,001
Proved Undeveloped Reserves:    
Beginning of year | MMcf 2,003 55
Extensions, discoveries and improved recovery | MMcf 335 1,994
Revisions of previous estimates | MMcf (1,153) 19
Converted to developed reserves | MMcf (476) (65)
End of year | MMcf 709 2,003
Total Proved Reserves at the End of the Year | MMcf 17,852 15,004