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