Other Comprehensive Income - Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income [Abstract]
Net unrealized losses from cash flow hedges, tax benefit	$ 1	$ 2

Accumulated other comprehensive income loss cumulative changes in net gain loss from cash flow hedges effect tax.

No definition available.

Name:	ck0000096271_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectTax
Namespace Prefix:	ck0000096271_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant