Rate and Other Regulatory Matters (Schedule of Regulatory Liabilities) (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory Liabilities
Regulatory liability, current	$ 269	$ 126
Regulatory liability, noncurrent	3,277	2,264
Total regulatory liabilities	3,546	2,390
Reserve For Refunds To Electric Utility Customers [Member]
Regulatory Liabilities
Regulatory liability, current	136	0
Regulatory liability, noncurrent	813	0
Monetization Of Guaranty Settlement [Member]
Regulatory Liabilities
Regulatory liability, current	67	61
Regulatory liability, noncurrent	1,003	1,037
Income Taxes Refundable Through Future Rates [Member]
Regulatory Liabilities
Regulatory liability, current	48	52
Regulatory liability, noncurrent	826	607
Other Regulatory Liability [Member]
Regulatory Liabilities
Regulatory liability, current	18	13
Regulatory liability, noncurrent	6	4
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities
Regulatory liability, noncurrent	552	541
Deferred Gains On Interest Rate Derivatives [Member]
Regulatory Liabilities
Regulatory liability, noncurrent	$ 77	$ 75