Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		27 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, authorized (in shares)	500,000,000		500,000,000	500,000,000
Common stock, issued (in shares)	309,752,140		309,752,140	309,752,140
Common stock, outstanding (in shares)	309,005,485		309,005,485	309,005,272
Common stock, par value (in usd per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Common stock, issued	$ 3		$ 3	$ 3
Additional paid-in capital	$ 13,667		$ 13,667	$ 13,660
Treasury stock, shares (in shares)	485,597		485,597	485,810
Convertible preferred stock, shares outstanding (in shares)	0		0	0
Issuances of common stock (in shares)	0	0
Release of common stock held in trust (in shares)		0
Treasury shares of common stock (in shares)	261,058		261,058
Repurchases of common stock (in shares)			0
Repurchases of common stock	$ 12		$ 12	$ 12
Iberdrola Renewables Holding, Inc
Class of Stock [Line Items]
Percentage of equity owned by parent	81.50%		81.50%	81.50%