Risk Management and Hedging Activities - Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Accumulated Other Comprehensive (Loss) Income	$ (7)		$ (7)			$ (7)
Trading and marketing gains (losses), net	11	$ (1)	156	$ 1
Commodity derivatives | Derivative Assets Not Designated As Hedging Instruments
Derivative [Line Items]
Realized gains	22	25	90	42
Unrealized gains (losses)	(11)	(26)	66	(41)
Trading and marketing gains (losses), net	11	(1)	156	1
Accumulated Net Gain (Loss) from Cash Flow Hedges
Derivative [Line Items]
Accumulated Other Comprehensive (Loss) Income	(7)	(8)	(7)	(8)	$ (7)	(7)	$ (8)	$ (8)
Accumulated Net Gain (Loss) from Cash Flow Hedges | Interest Rate Derivatives
Derivative [Line Items]
Accumulated Other Comprehensive (Loss) Income	(2)	(3)	(2)	(3)	(2)	(2)	(3)	(3)
Accumulated Net Gain (Loss) from Cash Flow Hedges | Commodity derivatives
Derivative [Line Items]
Accumulated Other Comprehensive (Loss) Income	(6)	(6)	(6)	(6)	(6)	(6)	(6)	(6)
Accumulated Net Gain (Loss) from Cash Flow Hedges | Foreign Currency Derivatives
Derivative [Line Items]
Accumulated Other Comprehensive (Loss) Income	1	$ 1	1	$ 1	$ 1	$ 1	$ 1	$ 1
Cash Flow Hedging [Member]
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	(1)		(1)
Cash Flow Hedging [Member] | Interest Rate Derivatives
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	(1)		(1)
Cash Flow Hedging [Member] | Commodity derivatives
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	0		0
Cash Flow Hedging [Member] | Foreign Currency Derivatives
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ 0		$ 0