Employee Benefits - Amounts recognized in the Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	$ 1,060	$ 858
Pension Plan | U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	1,034	836
Accrued benefit liability - current	(1)	(1)
Accrued benefit liability - non-current	(20)	(34)
Accumulated other comprehensive loss	1,236	1,256
Net amount recognized	2,249	2,057
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	(43)	(44)
Accrued benefit liability - non-current	(1,154)	(1,175)
Accumulated other comprehensive loss	1,706	1,618
Net amount recognized	509	399
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	(140)	(146)
Accumulated other comprehensive loss	452	394
Net amount recognized	$ 307	$ 243