Rate and Other Regulatory Matters - Transmission Projects (Narrative) (Detail) - DESC - Transmission Project $ in Millions	1 Months Ended	9 Months Ended
Jun. 30, 2020 USD ($) kV mi	Sep. 30, 2020 USD ($) NuclearPlant
Rate And Other Regulatory Matters [Line Items]
Number of nuclear plants under development \| NuclearPlant		2
Cost of anticipated project		$ 75
Transmission lines length of miles \| mi	28
Transmission lines capacity \| kV	230
Transmission lines project cost	$ 30

Carrying amount as of the balance sheet date of investments in power and distribution projects that are expected to be realized after one year or beyond the operating cycle, if longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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