Commitments And Contingent Liabilities (Guaranteed Obligations) (Detail) - Power [Member] - USD ($) $ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Loss Contingencies [Line Items] | ||
| Face Value of Outstanding Guarantees | $ 1,755 | $ 1,701 |
| Exposure under Current Guarantees | 161 | 153 |
| Letters of Credit Margin Posted | 109 | 103 |
| Letters of Credit Margin Received | 18 | 32 |
| Counterparty Cash Margin Deposited | 0 | 0 |
| Counterparty Cash Margin Received | (1) | (1) |
| Net Broker Balance Deposited (Received) | 122 | 147 |
| Other Letters of Credit | $ 61 | $ 61 |