Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - Pension Benefits [Member] $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Funded Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 65
Expected employer contributions in 2019	26
Unfunded Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	21
Expected employer contributions in 2019	$ 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