Employee Benefits - Schedule of changes in Level 3 fair value for U.K. Pension Plans (Details) - Pension Plan - U.K. - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Actual return on plan assets:
Balance at the beginning of the period	$ 5,959	$ 5,225
Foreign exchange	283	159
Balance at the end of the period	6,652	5,959
Significant Unobservable Inputs (Level 3)
Actual return on plan assets:
Balance at the beginning of the period	1,849	1,688
Relating to assets still held at the end of the year	13	113
Purchases, sales and settlements-net	682
Foreign exchange	81	48
Balance at the end of the period	$ 2,625	$ 1,849