Trust Investments (Narrative) (Detail)	9 Months Ended
Sep. 30, 2018 Facility
Power [Member]
Schedule of Trust Investments [Line Items]
Number of Nuclear Facilities	5

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration