Employee Benefits - Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
U.S.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	$ 1,812	$ 1,762
Prior service cost (income)	0	1
Net amount recognized	1,812	1,763
U.K.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	1,286	1,204
Prior service cost (income)	43	40
Net amount recognized	1,329	1,244
Other
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	521	460
Prior service cost (income)	(7)	(6)
Net amount recognized	$ 514	$ 454