Operating Revenue (Balance and Activity Related to Contract Costs Deferred as Regulatory Assets) (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2025	Dec. 31, 2024
Revenues [Abstract]
Beginning balance	$ 10	$ 11
Additional costs	4	0
Amortization	0	(1)
Ending balance	$ 14	$ 10

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 340
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479483/340-40-50-3

Name:	us-gaap_CapitalizedContractCostNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant