Financial Risk Management Activities (Narrative) (Detail) - Power [Member] - USD ($) $ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Derivatives, Fair Value [Line Items] | ||
| Fair Value of Derivatives with credit-risk related contingent features | $ 20 | $ 30 |
| Aggregate fair value of derivative contracts in a liability position that contains triggers for additional collateral | 9 | 13 |
| Additional collateral aggregate fair value | $ 11 | $ 17 |