Employee Benefits - Other Pre-tax Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive (Income) Loss (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (benefit)	$ 0	$ 230	$ 0
Reversal of amortization items
Prior service cost (benefit)	(61)	(48)	(39)
Total recognized in other comprehensive loss (income) (pre-tax)	(61)	182	(39)
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (benefit)	(22)	(8)	(544)
Reversal of amortization items
Prior service cost (benefit)	971	976	949
Total recognized in other comprehensive loss (income) (pre-tax)	$ 949	$ 968	$ 405

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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