Commitments And Contingent Liabilities (Face Value Of Outstanding Guarantees, Current Exposure And Margin Positions) (Detail) - Power [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Face Value of Outstanding Guarantees	$ 1,701	$ 1,806
Exposure under Current Guarantees	153	139
Letters of Credit Margin Posted	103	157
Letters of Credit Margin Received	32	99
Counterparty Cash Margin Deposited	0	0
Counterparty Cash Margin Received	(1)	(1)
Net Broker Balance Deposited (Received)	147	57
Other Letters of Credit	$ 61	$ 51