Postretirement Benefit Plans - Estimated Future Benefit Payments (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Company contributions	$ 0
Contributions expected to made by employer in 2018	5,000
Expected Future Benefit Payment [Abstract]
Payments in 2018	2,450
Payments in 2019	2,480
Payments in 2020	2,560
Payments in 2021	2,630
Payments in 2022	2,700
Payments in 2023 - 2027	14,200
Retiree Medical and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Company contributions	40	$ 36
Expected Future Benefit Payment [Abstract]
Payments in 2018	180
Payments in 2019	180
Payments in 2020	180
Payments in 2021	180
Payments in 2022	180
Payments in 2023 - 2027	$ 820

Amount of contribution received by defined benefit plan from employer which increases plan assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(ii)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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