Commitments And Contingent Liabilities (Minimum Fuel Purchase Requirements) (Detail) - Power [Member] $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Coverage percentage of nuclear fuel commitments of uranium, enrichment, and fabrication requirements	100.00%
Nuclear Fuel Uranium [Member]
Long-term Purchase Commitment [Line Items]
Total five-year minimum purchase requirements	$ 257
Nuclear Fuel Enrichment [Member]
Long-term Purchase Commitment [Line Items]
Total five-year minimum purchase requirements	328
Nuclear Fuel Fabrication [Member]
Long-term Purchase Commitment [Line Items]
Total five-year minimum purchase requirements	178
Natural Gas [Member]
Long-term Purchase Commitment [Line Items]
Total five-year minimum purchase requirements	963
Coal [Member]
Long-term Purchase Commitment [Line Items]
Total five-year minimum purchase requirements	$ 308