Employee Benefits - Amounts recognized in the Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	$ 1,280	$ 1,216
Pension Plan | U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	0	0
Accrued benefit liability - current	(52)	(50)
Accrued benefit liability - non-current	(1,052)	(1,076)
Accumulated other comprehensive loss	1,812	1,763
Net amount recognized	708	637
Pension Plan | U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	1,268	1,200
Accrued benefit liability - current	(1)	(1)
Accrued benefit liability - non-current	(21)	(19)
Accumulated other comprehensive loss	1,329	1,244
Net amount recognized	2,575	2,424
Pension Plan | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	0	0
Accrued benefit liability - current	(5)	(5)
Accrued benefit liability - non-current	(123)	(117)
Accumulated other comprehensive loss	514	454
Net amount recognized	$ 386	$ 332