Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - Pension Benefits [Member] $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($)
Funded Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 91
Unfunded Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 25

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

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration