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PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Components of Net Periodic Benefit Costs (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost   $ 194 $ 193 $ 224
Interest cost   287 373 372
Expected return on plan assets   (618) (540) (574)
Amortization of prior service costs (credits)   12 7 7
Special termination costs   0 14 31
One-time termination benefits   8 0 0
Pension & OPEB mark-to-market   463 656 227
Net periodic benefit costs (credits)   346 703 287
Net accelerated credits $ 18      
OPEB        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost   4 3 5
Interest cost   15 22 25
Expected return on plan assets   (33) (29) (31)
Amortization of prior service costs (credits)   (46) (36) (81)
Special termination costs   0 0 8
One-time termination benefits   0 0 0
Pension & OPEB mark-to-market   14 20 (82)
Net periodic benefit costs (credits)   $ (46) $ (20) $ (156)