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Commitments And Contingent Liabilities (Regulatory Proceedings) (Detail) - Power [Member] - FERC Compliance matters [Domain]
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
Loss Contingencies [Line Items]  
Loss Contingency. Loss in Period $ 5
Loss Contingency, Estimate of Possible Loss 40
Loss Contingency Accrual $ 40