Risk Management and Hedging Activities - Summary of Gross and Net Amounts of Derivative Instruments (Detail) - Commodity derivatives - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Offsetting Assets [Line Items]
Gross Amounts of Assets Presented in the Balance Sheet	$ 305	$ 34
Amounts Not Offset in the Balance Sheet - Financial Instruments	0	0
Net Amount	305	34
Offsetting Liabilities [Line Items]
Gross Amounts of Liabilities Presented in the Balance Sheet	(238)	(78)
Amounts Not Offset in the Balance Sheet - Financial Instruments	0	0
Net Amount	$ (238)	$ (78)