Rate and Other Regulatory Matters (Schedule of Regulatory Assets) (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory Assets
Regulatory assets, current	$ 259	$ 223
Regulatory assets, noncurrent	3,759	4,046
Total regulatory assets	4,018	4,269
NND Project Costs [Member]
Regulatory Assets
Regulatory assets, current	138	127
Regulatory assets, noncurrent	2,572	2,641
Deferred Employee Benefit Plan Costs [Member]
Regulatory Assets
Regulatory assets, current	16	16
Regulatory assets, noncurrent	230	256
Other Unrecovered Plant [Member]
Regulatory Assets
Regulatory assets, current	14	14
Regulatory assets, noncurrent	73	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	75	52
Regulatory assets, noncurrent	117	123
Asset Retirement Obligation Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	324	380
Deferred Losses On Interest Rate Derivatives [Member]
Regulatory Assets
Regulatory assets, noncurrent	308	442
Environmental Remediation Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	21	24
Deferred Storm Damage Costs [Member]
Regulatory Assets
Regulatory assets, noncurrent	34	35
Deferred Transmission Operating Costs [Member] [Domain]
Regulatory Assets
Regulatory assets, noncurrent	$ 27	$ 15