Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income attributable to AVANGRID	$ 240	$ 217
Denominator:
Weighted average number of shares outstanding - basic (in shares)	309,491,082	309,491,082
Weighted average number of shares outstanding - diluted (in shares)	309,623,573	309,712,308
Earnings per share attributable to AVANGRID
Earnings Per Common Share, Basic (in usd per share)	$ 0.78	$ 0.70
Earnings Per Common Share, Diluted (in usd per share)	$ 0.78	$ 0.70