Pension and Other Postretirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2020	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Impact of curtailment		$ (69)
Postretirement medical plan
Defined Benefit Plan Disclosure [Line Items]
Impact of curtailment	$ 69	$ 69

Amount of gain (loss) recognized in net periodic benefit (cost) credit from settlement and curtailment.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)(7)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(7)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=123453770&loc=SL108413299-114919

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