Employee Benefit Plans (Narrative) (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2021
Compensation And Retirement Disclosure [Abstract]
Defined benefit plan, expected future employer contributions, next fiscal year, description	no	no

Defined benefit plan, expected future employer contributions, remainder of fiscal year, description.

No definition available.

Name:	ck91882_DefinedBenefitPlanExpectedFutureEmployerContributionsRemainderOfFiscalYearDescription
Namespace Prefix:	ck91882_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration