Background and Nature Of Operations (Detail)	Sep. 30, 2020
Avangrid \| Iberdrola S.A.
Nature Of Business [Line Items]
Percentage of equity owned by parent	81.50%

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

No definition available.

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant