Derivatives and Concentration of Credit Risk Derivatives and Concentration of Credit Risk - Fair Values and Gains (Losses) (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
DerivativeGainLoss [Line Items]
Net gain (loss) on derivatives		$ (199)	$ 175	$ 78	$ (207)	$ 443	$ (139)	$ (154)	$ (69)	$ (153)	$ 81	$ 3
Energy Related Derivative
DerivativeGainLoss [Line Items]
Net gain (loss) on derivatives	[1]									(150)	78	3
Derivative, Cost of Hedge											237
Derivative, Cash Received on Hedge										12		4
Derivatives Related to Physical Marketing Agreements
DerivativeGainLoss [Line Items]
Net gain (loss) on derivatives	[2]									(3)	3	0
Derivative, Cost of Hedge										$ 1	$ 1	$ 1

[1] Includes settlements totaling $12 million for the year ended December 31, 2019, payments totaling $237 million for the year ended December 31, 2018, and settlements totaling $4 million for the year ended December 31,

[2] Includes payments totaling less than $1 million for the years ended December 31, 2019, 2018 and 2017.