Segment Information - Reconciliation of Adjusted Net Income to Net Income (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting [Abstract]
Adjusted Net Income Attributable to Avangrid, Inc.	$ 236	$ 219
Adjustments:
Mark-to-market earnings - Renewables	18	3
Restructuring charges	(3)	0
Accelerated depreciation from repowering	(10)	(5)
Income tax impact of adjustments	(2)	0
Net Income Attributable to Avangrid, Inc.	$ 240	$ 217

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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