Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income attributable to AVANGRID	$ 120	$ 102	$ 359	$ 314
Denominator:
Weighted average number of shares outstanding - basic	309,520,718	309,527,868	309,514,836	309,533,042
Weighted average number of shares outstanding - diluted	309,826,185	309,683,965	309,799,839	309,689,138
Earnings per share attributable to AVANGRID
Earnings Per Common Share, Basic	$ 0.39	$ 0.33	$ 1.16	$ 1.01
Earnings Per Common Share, Diluted	$ 0.39	$ 0.33	$ 1.16	$ 1.01