Derivative Instruments and Hedging - Summary of Unrealized Gains and Losses from Fair Value Adjustments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Derivative assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on derivatives	$ 0	$ (1)
Derivative liabilities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on derivatives	(3)	(1)
Electricity | Regulatory assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Loss or Gain Recognized in Regulatory Assets/Liabilities	34		$ 24
Unrealized gain (loss) on derivatives	21	4
Natural Gas | Regulatory assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Loss or Gain Recognized in Regulatory Assets/Liabilities	0		$ 4
Unrealized gain (loss) on derivatives	$ 5	$ 0