Financial Risk Management Activities (Schedule Of Derivative Instruments Fair Value In Balance Sheets) (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		$ 121	$ 146
Derivative Contracts, Current Assets		43	29
Derivative Contracts, Noncurrent Assets		48	7
Total Mark-to-Market Derivative Assets		91	36
Derivative Contracts, Current Liabilities		(10)	(16)
Derivative Contracts, Noncurrent Liabilities		(2)	(5)
Total Mark-to-Market Derivative (Liabilities)		(12)	(21)
Net Mark-to-Market Derivative Assets (Liabilities)		79	15
Derivative, Fair Value, Amount Offset Against Collateral, Net		(2)	44
Power [Member]
Derivatives, Fair Value [Line Items]
Derivative Contracts, Current Assets	[1]	43	29
Derivative Contracts, Noncurrent Assets	[1]	48	7
Total Mark-to-Market Derivative Assets	[1]	91	36
Derivative Contracts, Current Liabilities	[1]	(10)	(16)
Derivative Contracts, Noncurrent Liabilities	[1]	(2)	(5)
Total Mark-to-Market Derivative (Liabilities)	[1]	(12)	(21)
Net Mark-to-Market Derivative Assets (Liabilities)	[1]	79	15
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2]	(2)	44
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net			19
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net			(3)
Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net			(3)
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net			28
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net			47
Energy-Related Contracts [Member] | Not Designated as Hedging Instrument [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative Contracts, Current Assets	[1]	366	391
Derivative Contracts, Noncurrent Assets	[1]	156	78
Total Mark-to-Market Derivative Assets	[1]	522	469
Derivative Contracts, Current Liabilities	[1]	(332)	(403)
Derivative Contracts, Noncurrent Liabilities	[1]	(109)	(95)
Total Mark-to-Market Derivative (Liabilities)	[1]	(441)	(498)
Net Mark-to-Market Derivative Assets (Liabilities)	[1]	81	(29)
Energy-Related Contracts [Member] | Other Noncurrent Liabilities [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2]	107	90
Energy-Related Contracts [Member] | Other Current Assets [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2]	(323)	(362)
Energy-Related Contracts [Member] | Other Noncurrent Assets [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2]	(108)	(71)
Energy-Related Contracts [Member] | Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[3],[4]	(431)	(433)
Energy-Related Contracts [Member] | Assets [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2],[3],[4]	(431)	(433)
Energy-Related Contracts [Member] | Assets [Member] | PSE And G [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[3],[4]		0
Energy-Related Contracts [Member] | Other Current Liabilities [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2]	322	387
Energy-Related Contracts [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[3],[4]	429	477
Energy-Related Contracts [Member] | Other Liabilities [Member] | Power [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[1],[2],[3],[4]	$ 429	477
Energy-Related Contracts [Member] | Other Liabilities [Member] | PSE And G [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	[3],[4]		$ 0

[1] 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.

[2] 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.

[3] Level 1—These contracts represent natural gas futures contracts executed on NYMEX, and are being valued solely on settled pricing inputs which come directly from the exchange.Level 2—Fair values for energy-related contracts are obtained primarily using a market-based approach. Most derivative contracts (forward purchase or sale contracts and swaps) are valued using settled prices from similar assets and liabilities from an exchange, such as NYMEX, ICE and Nodal Exchange, or auction prices. Prices used in the valuation process are also corroborated independently by management to determine that values are based on actual transaction data or, in the absence of transactions, bid and offers for the day. Examples may include certain exchange and non-exchange traded capacity and electricity contracts and natural gas physical or swap contracts based on market prices, basis adjustments and other premiums where adjustments and premiums are not considered significant to the overall inputs.Level 3—Unobservable inputs are used for the valuation of certain contracts. See “Additional Information Regarding Level 3 Measurements” below for more information on the utilization of unobservable inputs.

[4] 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 in 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 of December 31, 2017 were netted against the corresponding net derivative contract positions. The $(2) million of cash collateral as of March 31, 2018 was netted against assets. Of the $44 million of cash collateral as of December 31, 2017, $(3) million was netted against assets and $47 million was netted against liabilities.