Rate and Other Regulatory Matters (Schedule of Regulatory Assets) (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets
Regulatory assets, current	$ 322	$ 224
Regulatory assets, noncurrent	3,970	4,060
Total regulatory assets	4,292	4,284
NND Project Costs [Member]
Regulatory Assets
Regulatory assets, current	138	127
Regulatory assets, noncurrent	2,537	2,641
Income Taxes Recoverable Through Future Rates [Member]
Regulatory Assets
Regulatory assets, current	52	0
Deferred Employee Benefit Plan Costs [Member]
Regulatory Assets
Regulatory assets, current	13	16
Regulatory assets, noncurrent	205	256
Other Unrecovered Plant [Member]
Regulatory Assets
Regulatory assets, current	14	14
Regulatory assets, noncurrent	70	79
Demand Side Management Programs [Member]
Regulatory Assets
Regulatory assets, current	16	14
Regulatory assets, noncurrent	53	51
Other Regulatory Assets [Member]
Regulatory Assets
Regulatory assets, current	89	53
Regulatory assets, noncurrent	113	123
Asset Retirement Obligation Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	315	380
Deferred Losses On Interest Rate Derivatives [Member]
Regulatory Assets
Regulatory assets, noncurrent	310	442
Cost of Reacquired Debt [Member]
Regulatory Assets
Regulatory assets, noncurrent	271	14
Environmental Remediation Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	21	24
Deferred Storm Damage Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	43	35
Deferred Transmission Operating Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	$ 32	$ 15