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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Company Before Tax Obligation      
Balance at January 1 $ 14,214 $ 14,243 $ 15,642
Liabilities incurred 96 684 204
Liabilities settled (830) (1,721) (1,658)
Accretion expense 654 668 749
Revisions in estimated cash flows (84) 340 (694)
Balance at December 31 14,050 $ 14,214 $ 14,243
Long-term portion of the company's before-tax asset retirement obligations $ 12,957