Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - Pension Benefits [Member] $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Funded Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 161
Expected employer contributions in 2018	1
Unfunded Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	36
Expected employer contributions in 2018	$ 26

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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration