Operating Revenue (Balance and Activity Related to Contract Costs Deferred as Regulatory Assets) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Revenues [Abstract]
Beginning balance	$ 13	$ 15
Amortization	(1)	(2)
Ending balance	$ 12	$ 13

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

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