Trust Investments (Narrative) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) Facility
Power [Member]
Schedule of Trust Investments [Line Items]
Number of Nuclear Facilities | Facility	5
Debt Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Trust Investments [Line Items]
After tax amount of net unrealized gains recognized in AOCI	$ 3
Debt Securities [Member] | Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
After tax amount of net unrealized gains recognized in AOCI	2
Equity Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Trust Investments [Line Items]
Unrealized Gains (Losses) on Equity Securities still held	88
Equity Securities [Member] | Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Unrealized Gains (Losses) on Equity Securities still held	$ 3