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Acquisitions and Dispositions (Schedule of Results of Operations Reported As Discontinued Operations) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Nov. 06, 2019
Dec. 31, 2018
Dec. 31, 2017
East Ohio      
Business Acquisition [Line Items]      
Operating revenue $ 594 $ 729 $ 728
Depreciation and amortization 73 76 71
Other operating expenses 399 444 428
Other income 61 72 50
Interest and related charges 33 37 33
Income tax expense (benefit) 26 53 86
Net income (loss) from discontinued operations 124 191 160
DGP      
Business Acquisition [Line Items]      
Operating revenue 125 220 114
Depreciation and amortization 4 15 15
Impairment of assets and other charges   219  
Other operating expenses 97 206 91
Income tax expense (benefit) 7 (53) 5
Net income (loss) from discontinued operations $ 17 $ (167) $ 3