Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income attributable to AVANGRID	$ 87	$ 150	$ 415	$ 477
Denominator:
Weighted average number of shares outstanding - basic (in shares)	309,491,082	309,491,082	309,496,234	309,491,082
Weighted average number of shares outstanding - diluted (in shares)	309,550,126	309,517,778	309,554,838	309,512,301
Earnings per share attributable to AVANGRID
Earnings Per Common Share, Basic (in usd per share)	$ 0.28	$ 0.48	$ 1.34	$ 1.54
Earnings Per Common Share, Diluted (in usd per share)	$ 0.28	$ 0.48	$ 1.34	$ 1.54