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, 2018	Dec. 31, 2017
Actual return on plan assets:
Balance at the beginning of the period	$ 5,906	$ 5,675
Foreign exchange	317	(513)
Balance at the end of the period	5,225	5,906
Annuities
Actual return on plan assets:
Balance at the beginning of the period	1,909
Balance at the end of the period	1,688	1,909
Significant Unobservable Inputs (Level 3)
Actual return on plan assets:
Balance at the beginning of the period	1,909
Balance at the end of the period	1,688	1,909
Significant Unobservable Inputs (Level 3) | Annuities
Actual return on plan assets:
Balance at the beginning of the period	1,909	1,773
Relating to assets still held at the end of the year	(122)	(66)
Purchases, sales and settlements—net	7	45
Foreign exchange	(106)	157
Balance at the end of the period	$ 1,688	$ 1,909