Employee Benefits - Schedule of changes in Level 3 fair value for U.K. Pension Plans (Details) - U.K. - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Actual return on plan assets:
Balance at the beginning of the period	$ 5,903	$ 6,224
Foreign exchange	(1,127)	(257)
Balance at the end of the period	5,675	5,903
Global
Actual return on plan assets:
Balance at the beginning of the period	360
Balance at the end of the period	365	360
Annuities
Actual return on plan assets:
Balance at the beginning of the period	827
Balance at the end of the period	1,773	827
Alternative investments
Actual return on plan assets:
Balance at the beginning of the period	717
Balance at the end of the period	773	717
Pooled funds - Fixed income securities
Actual return on plan assets:
Balance at the beginning of the period	283
Balance at the end of the period	223	283
Significant Unobservable Inputs (Level 3)
Actual return on plan assets:
Balance at the beginning of the period	827
Balance at the end of the period	1,773	827
Significant Unobservable Inputs (Level 3) | Global
Actual return on plan assets:
Balance at the beginning of the period	0
Balance at the end of the period	0	0
Significant Unobservable Inputs (Level 3) | Annuities
Actual return on plan assets:
Balance at the beginning of the period	827	836
Relating to assets still held at the end of the year	7	(32)
Purchases, sales and settlements—net	1,248	58
Foreign exchange	(309)	(35)
Balance at the end of the period	1,773	827
Significant Unobservable Inputs (Level 3) | Alternative investments
Actual return on plan assets:
Balance at the beginning of the period	0
Balance at the end of the period	0	0
Significant Unobservable Inputs (Level 3) | Pooled funds - Fixed income securities
Actual return on plan assets:
Balance at the beginning of the period	0
Balance at the end of the period	$ 0	$ 0