Employee Benefits - Amounts recognized in the Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	$ 1,133	$ 1,060
Pension Plan | U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	1,113	1,034
Accrued benefit liability - current	(1)	(1)
Accrued benefit liability - non-current	(16)	(20)
Accumulated other comprehensive loss	1,136	1,236
Net amount recognized	2,232	2,249
Pension Plan | U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	0	0
Accrued benefit liability - current	(46)	(43)
Accrued benefit liability - non-current	(1,035)	(1,154)
Accumulated other comprehensive loss	1,708	1,706
Net amount recognized	627	509
Pension Plan | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	0	0
Accrued benefit liability - current	(5)	(5)
Accrued benefit liability - non-current	(111)	(140)
Accumulated other comprehensive loss	433	452
Net amount recognized	$ 317	$ 307