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POSTRETIREMENT BENEFIT PLANS - Effect on Operations of Pension Plans (Details) - Principal pension plans - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]        
Service cost for benefits earned $ 57 $ 156 $ 179 $ 501
Prior service cost amortization 8 36 22 110
Expected return on plan assets (763) (747) (2,288) (2,243)
Interest cost on benefit obligations 488 588 1,464 1,764
Net actuarial loss amortization 871 850 2,613 2,549
Plans cost (income) $ 661 $ 883 $ 1,990 $ 2,681