Risk Management and Hedging Activities - Summary of Gross and Net Amounts of Derivative Instruments (Detail) - Commodity derivatives - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Offsetting Assets [Line Items]
Gross Amounts of Assets Presented in the Balance Sheet	$ 127	$ 34
Derivative Asset, Not Offset in Balance Sheet	0	0
Net Amount	127	34
Offsetting Liabilities [Line Items]
Gross Amounts of Liabilities Presented in the Balance Sheet	103	78
Derivative Liability, Not Offset in Balance Sheet	0	0
Net Amount	$ 103	$ 78