Trust Investments (Narrative) (Detail) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($) Facility
Power [Member]
Schedule of Trust Investments [Line Items]
Number of Nuclear Facilities \| Facility	5
Total Debt Securities [Member] \| Nuclear Decommissioning Trust (NDT) Fund [Member] \| Power [Member]
Schedule of Trust Investments [Line Items]
After tax amount of net unrealized gains recognized in AOCI	$ (10)
Equity Securities [Member] \| Nuclear Decommissioning Trust (NDT) Fund [Member] \| Power [Member]
Schedule of Trust Investments [Line Items]
Unrealized Gains (Losses) on Equity Securities still held	(10)
Equity Securities [Member] \| Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Unrealized Gains (Losses) on Equity Securities still held	$ (1)

Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities, attributable to parent entity. Excludes amounts related to other than temporary impairment (OTTI) loss.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569643-111683

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84161450&loc=SL4573702-111684

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569616-111683

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration