Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net		$ (2)		$ 44
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(1)	$ 22
Net assets measured at fair value on a recurring basis		2,400	2,500
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3		7	3
Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2]	(2)		$ 44
Net Derivative Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	[3]	0	(1)
Net Derivative Assets [Member] | Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	[3]	$ 0	$ (1)

[1] Represents the netting of fair value balances with the same counterparty (where the right of offset exists) and the application of collateral. All cash collateral received or posted that has been allocated to derivative positions, where the right of offset exists, has been offset on the Condensed Consolidated Balance Sheets. As of March 31, 2018 and December 31, 2017, Power had net cash collateral/margin payments to counterparties of $121 million and $146 million, respectively. Of these net cash/collateral margin payments $(2) million as of March 31, 2018 and $44 million as December 31, 2017 were netted against the corresponding net derivative contract positions. The $(2) million as of March 31, 2018 was netted against current assets. Of the $44 million as of December 31, 2017, $(3) million was netted against current assets, $28 million was netted against current liabilities, and $19 million was netted against noncurrent liabilities.

[2] Substantially all of Power’s derivative instruments are contracts subject to master netting agreements. Contracts not subject to master netting or similar agreements are immaterial and did not have any collateral posted or received as of March 31, 2018 and December 31, 2017.

[3] During the three months ended March 31, 2017, $(1) million of net derivatives were transferred from Level 2 to Level 3.