Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=120401555&loc=SL114874292-224272

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Name:	us-gaap_RevenueFromRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Related Party Transactions - Schedule of Related Party Transactions (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Iberdrola Renovables Energía, S.L.
Related Party Transaction [Line Items]
Purchases From	$ (2)	$ (1)	$ (6)	$ (10)
Iberdrola Financiación, S.A.
Related Party Transaction [Line Items]
Purchases From	(3)	(1)	(5)	(2)
Iberdrola, S.A.
Related Party Transaction [Line Items]
Purchases From	(10)	(8)	(31)	(28)
Vineyard Wind
Related Party Transaction [Line Items]
Sales To	3	4	7	11
Other
Related Party Transaction [Line Items]
Sales To	0	0	0	1
Purchases From	(1)	$ (1)	(2)	$ (2)
Iberdrola Solutions
Related Party Transaction [Line Items]
Sales To	$ 2		$ 2