Segment Information - Reconciliation of Adjusted Net Income to Net Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting [Abstract]
Adjusted Net Income Attributable to Avangrid, Inc.	$ 100	$ 123	$ 434	$ 442
Adjustments:
Mark-to-market earnings - Renewables	(7)	42	9	66
Restructuring charges	(1)	(2)	(5)	(4)
Accelerated depreciation from repowering	(3)	(5)	(9)	(15)
Impact of COVED -19	(8)	0	(21)	0
Income tax impact of adjustments	5	(9)	7	(12)
Net Income Attributable to Avangrid, Inc.	$ 87	$ 150	$ 415	$ 477