Variable Interest Entities - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($) MW
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power | $	$ 49	$ 59	$ 46
Power Purchase Agreements [Member] | Variable Interest Entity Not Primary Beneficiary [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power | $	$ 25	$ 15	$ 16
Minimum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range		121	121
Maximum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range		250	250
PPAs range	360