Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Parenthetical) (Detail)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Minimum [Member] \| Storm Reserve [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory asset recovery period	12 months	12 months

Recovery period of regulatory assets for which return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	ck0000096271_RecoveryPeriodOfRegulatoryAssetsForWhichReturnOnInvestmentDuringRecoveryPeriodIsProvided
Namespace Prefix:	ck0000096271_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration