Trust Investments (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions |
Mar. 31, 2018
USD ($)
|
|---|---|
| Rabbi Trust [Member] | |
| Schedule of Trust Investments [Line Items] | |
| Less than one year | $ 1 |
| 1 - 5 years | 36 |
| 6 - 10 years | 29 |
| 11 - 15 years | 6 |
| 16 - 20 years | 17 |
| Over 20 years | 113 |
| Total Available-for-Sale Debt Securities | 202 |
| Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | |
| Schedule of Trust Investments [Line Items] | |
| Less than one year | 15 |
| 1 - 5 years | 300 |
| 6 - 10 years | 201 |
| 11 - 15 years | 40 |
| 16 - 20 years | 73 |
| Over 20 years | 335 |
| Total Available-for-Sale Debt Securities | $ 964 |