Annual report pursuant to Section 13 and 15(d)

PENSION AND OTHER POSTRETIREMENT BENEFITS - (Details 5 ) - Level 3 Fair Value Reconciliation

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PENSION AND OTHER POSTRETIREMENT BENEFITS - (Details 5 ) - Level 3 Fair Value Reconciliation - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
UNITED STATES      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets $ 3,429 $ 3,357 $ 2,937
UNITED STATES | Fair Value, Inputs, Level 3      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 431 371 316
Unrealized gains (losses) on assets still held at the reporting date 23 29  
Purchases, sales and settlements, net 37 26  
UNITED STATES | Fair Value, Inputs, Level 3 | Private Markets      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 359 299 247
Unrealized gains (losses) on assets still held at the reporting date 21 24  
Purchases, sales and settlements, net 39 28  
UNITED STATES | Fair Value, Inputs, Level 3 | Real Assets      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 72 72 69
Unrealized gains (losses) on assets still held at the reporting date 2 5  
Purchases, sales and settlements, net (2) (2)  
UNITED KINGDOM      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 2,337 2,010 1,782
UNITED KINGDOM | Insurance      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 556 476  
UNITED KINGDOM | Fair Value, Inputs, Level 3      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 838 735 686
Unrealized gains (losses) on assets still held at the reporting date 100 53  
Purchases, sales and settlements, net 3 (4)  
UNITED KINGDOM | Fair Value, Inputs, Level 3 | Private Markets      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 251 224 187
Unrealized gains (losses) on assets still held at the reporting date 22 14  
Purchases, sales and settlements, net 5 23  
UNITED KINGDOM | Fair Value, Inputs, Level 3 | Real Assets      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 31 35 57
Unrealized gains (losses) on assets still held at the reporting date (2) 5  
Purchases, sales and settlements, net (2) (27)  
UNITED KINGDOM | Fair Value, Inputs, Level 3 | Insurance      
Summary of changes in the fair value of level 3 assets      
Fair value of plan assets 556 476 $ 442
Unrealized gains (losses) on assets still held at the reporting date 80 34  
Purchases, sales and settlements, net $ 0 $ 0