Commitments And Contingent Liabilities (Guaranteed Obligations) (Detail) - Power [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Face Value of Outstanding Guarantees	$ 1,854	$ 1,772
Exposure under Current Guarantees	150	198
Letters of Credit Margin Posted	105	115
Letters of Credit Margin Received	29	26
Counterparty Cash Margin Deposited	0	0
Counterparty Cash Margin Received	(1)	(10)
Net Broker Balance Deposited (Received)	204	403
Other Letters of Credit	$ 51	$ 52