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SUPPLEMENTARY OIL & GAS INFORMATION (Unaudited) - Proved and Proved Developed Natural Gas Reserve Quantities (Details) - Natural Gas
 12 Months Ended
Dec. 31, 2018
MMBbls
Bcf
Dec. 31, 2017
MMBbls
Bcf
Dec. 31, 2016
MMBbls
Bcf
Dec. 31, 2015
Bcf
Proved Developed And Undeveloped Reserves [Roll Forward]        
Proved reserves, net, beginning balance 5,240 4,644 4,582  
Extensions and discoveries (in Bcf) 90 261 176  
Improved recovery (in Bcf) 414 179 169  
Purchases of reserves in place (in Bcf) 67 106 85  
Sales of reserves in place (in Bcf) (3) 0 5  
Production (in Bcf) (542) (579) (596)  
Economic revisions due to prices (in Bcf) 748 (407) 581  
Revisions of prior estimates (in Bcf) 164 (222) (814)  
Proved reserves, net, ending balance 4,354 5,240 4,644  
Net proved developed reserves (in Bcf) 2,417 3,112 2,841 2,924
Change in proved developed and undeveloped reserves, net (in Bcf) | MMBbls 886 596 62  
North America        
Proved Developed And Undeveloped Reserves [Roll Forward]        
Proved reserves, net, beginning balance 5,199 4,594 4,523  
Extensions and discoveries (in Bcf) 90 261 176  
Improved recovery (in Bcf) 414 179 166  
Purchases of reserves in place (in Bcf) 67 106 85  
Sales of reserves in place (in Bcf) 3 0 5  
Production (in Bcf) (523) (558) (571)  
Economic revisions due to prices (in Bcf) 746 (403) 572  
Revisions of prior estimates (in Bcf) 192 (214) (792)  
Proved reserves, net, ending balance 4,306 5,199 4,594  
Net proved developed reserves (in Bcf) 2,382 3,081 2,805 2,883
North Sea        
Proved Developed And Undeveloped Reserves [Roll Forward]        
Proved reserves, net, beginning balance 25 25 38  
Extensions and discoveries (in Bcf) 0 0 0  
Improved recovery (in Bcf) 0 0 0  
Purchases of reserves in place (in Bcf) 0 0 0  
Sales of reserves in place (in Bcf) 0 0 0  
Production (in Bcf) (11) (14) (14)  
Economic revisions due to prices (in Bcf) 0 (5) 10  
Revisions of prior estimates (in Bcf) (13) (9) (11)  
Proved reserves, net, ending balance 27 25 25  
Net proved developed reserves (in Bcf) 23 22 18 26
Offshore Africa        
Proved Developed And Undeveloped Reserves [Roll Forward]        
Proved reserves, net, beginning balance 16 25 21  
Extensions and discoveries (in Bcf) 0 0 0  
Improved recovery (in Bcf) 0 0 3  
Purchases of reserves in place (in Bcf) 0 0 0  
Sales of reserves in place (in Bcf) 0 0 0  
Production (in Bcf) (8) (7) (11)  
Economic revisions due to prices (in Bcf) 2 1 (1)  
Revisions of prior estimates (in Bcf) (15) 1 (11)  
Proved reserves, net, ending balance 21 16 25  
Net proved developed reserves (in Bcf) 12 9 18 15