Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Defined Benefit Plan Disclosure [Line Items] | ||
| Non-Operating Pension and OPEB Credits (Costs) | $ 19 | $ 0 |
| Pension Benefits [Member] | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Service Cost | 32 | 29 |
| Interest Cost | 52 | 51 |
| Expected Return on Plan Assets | (110) | (98) |
| Amortization of Prior Service Cost | (4) | (5) |
| Amortization of Actuarial Loss | 21 | 24 |
| Non-Operating Pension and OPEB Credits (Costs) | (41) | (28) |
| Total Benefit Costs | (9) | 1 |
| OPEB [Member] | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Service Cost | 4 | 4 |
| Interest Cost | 16 | 16 |
| Expected Return on Plan Assets | (10) | (8) |
| Amortization of Prior Service Cost | 0 | (3) |
| Amortization of Actuarial Loss | 16 | 13 |
| Non-Operating Pension and OPEB Credits (Costs) | 22 | 18 |
| Total Benefit Costs | $ 26 | $ 22 |