Employee Benefits - Reconciliation of the changes in the benefit obligations and fair value of assets and a statement of the funded status (Details) - Pension Plan £ in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2017 USD ($)	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
U.K.
Change in projected benefit obligation
Balance at the beginning of the Period					$ 4,893		$ 4,874
Service cost					0		0	$ 0
Interest cost					109		123	158
Plan amendment					13		0
Settlements		£ (42)	$ (61)		(176)		(496)
Actuarial loss (gain)					(297)		100
Benefit payments					(160)		(146)
Foreign currency impact					(253)		438
Balance at the end of the period	$ 4,893				4,129		4,893	4,874
Accumulated benefit obligation at end of year	4,893				4,129		4,893
Change in fair value of plan assets
Balance at the beginning of the period					5,906		5,675
Actual return on plan assets					(125)		274
Employer contributions					97		86
Settlements				£ (139)	(176)	£ (371)	(496)
Benefit payments		£ (116)	$ (159)		(160)		(146)
Foreign currency impact					(317)		513
Balance at the end of the period	5,906				5,225		5,906	5,675
Market related value at end of year	5,906				5,225		5,906
Funded status	1,013				1,096		1,013
Unrecognized prior-service cost	19				30		19
Unrecognized loss	1,217				1,106		1,217
Net amount recognized	2,249				2,232		2,249
U.S.
Change in projected benefit obligation
Balance at the beginning of the Period					3,155		2,902
Service cost					0		0	0
Interest cost					99		96	111
Plan amendment					0		0
Settlements	(158)				0		0
Actuarial loss (gain)					(221)		309
Benefit payments					(156)		(152)
Foreign currency impact					0		0
Balance at the end of the period	3,155				2,877		3,155	2,902
Accumulated benefit obligation at end of year	3,155				2,877		3,155
Change in fair value of plan assets
Balance at the beginning of the period					1,958		1,683
Actual return on plan assets					(141)		308
Employer contributions					135		119
Settlements					0		0
Benefit payments	(281)				(156)		(152)
Foreign currency impact					0		0
Balance at the end of the period	1,958				1,796		1,958	1,683
Market related value at end of year	1,926				1,981		1,926
Funded status	(1,197)				(1,081)		(1,197)
Unrecognized prior-service cost	5				3		5
Unrecognized loss	1,701				1,705		1,701
Net amount recognized	509				627		509
Other
Change in projected benefit obligation
Balance at the beginning of the Period					1,401		1,227
Service cost					0		0	0
Interest cost					27		26	29
Plan amendment					0		0
Settlements					0		0
Actuarial loss (gain)					(47)		49
Benefit payments					(43)		(39)
Foreign currency impact					(67)		138
Balance at the end of the period	1,401				1,271		1,401	1,227
Accumulated benefit obligation at end of year	1,373				1,247		1,373
Change in fair value of plan assets
Balance at the beginning of the period					1,256		1,076
Actual return on plan assets					(19)		70
Employer contributions					20		21
Settlements					0		0
Benefit payments					(43)		(39)
Foreign currency impact					(59)		128
Balance at the end of the period	1,256				1,155		1,256	$ 1,076
Market related value at end of year	1,256				1,155		1,256
Funded status	(145)				(116)		(145)
Unrecognized prior-service cost	(7)				(7)		(7)
Unrecognized loss	459				440		459
Net amount recognized	$ 307				$ 317		$ 307