Acquisitions and Dispositions (Schedule of Results of Operations of Disposal Groups Reported As Discontinued Operations) (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of assets and other charges			$ 2,105		$ 1,520		$ 12
Asset retirement obligations			5,583	[1]	5,184	[1]	2,444
Gains on sales of assets			$ 61		$ 152		265
GT&S Transaction
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Charge related to impairment of certain gathering and processing assets							219
After-tax charge related to the impairment of gathering and processing assets							165
Charge for disallowance of FERC-regulated plant							127
After-tax charge for disallowance of FERC-regulated plant							92
Loss associated with cash flow hedges of debt-related items	$ 237
Loss associated with cash flow hedges of debt-related items after tax	$ 178
GT&S Transaction | Marcellus Shale and Utica and Point Pleasant Shale
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Gains on sales of assets							115
Gains on sales of assets after tax							83
GT&S Transaction | FERC-regulated
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of assets and other charges							37
Asset impairment charges after tax							$ 28
Supply Header Project | GT&S Transaction
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of assets and other charges		$ 482
Asset impairment charges after tax		359
Atlantic Coast Pipeline Project | GT&S Transaction
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Asset retirement obligations		$ 75

[1] Includes $394 million and $179 million reported in other current liabilities at December 31, 2019 and 2020, respectively.