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, 2017	Dec. 31, 2016
Actual return on plan assets:
Balance at the beginning of the period	$ 5,675	$ 5,903
Foreign exchange	(513)	1,127
Balance at the end of the period	5,906	5,675
Annuities
Actual return on plan assets:
Balance at the beginning of the period	1,773
Balance at the end of the period	1,909	1,773
Significant Unobservable Inputs (Level 3)
Actual return on plan assets:
Balance at the beginning of the period	1,773
Balance at the end of the period	1,909	1,773
Significant Unobservable Inputs (Level 3) | Annuities
Actual return on plan assets:
Balance at the beginning of the period	1,773	827
Relating to assets still held at the end of the year	(66)	7
Purchases, sales and settlements—net	45	1,248
Foreign exchange	157	(309)
Balance at the end of the period	$ 1,909	$ 1,773