Trust Investments (Narrative) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Trust Investments [Line Items]
Decommissioning Liability, Noncurrent	$ 708
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Minimum [Member]
Schedule of Trust Investments [Line Items]
Decommissioning Costs Including Contingencies	2,800
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Maximum [Member]
Schedule of Trust Investments [Line Items]
Decommissioning Costs Including Contingencies	3,000
Debt Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Trust Investments [Line Items]
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent	(12)
Debt Securities [Member] | Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent	$ (4)