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 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
U.K.
Change in projected benefit obligation
Balance at the beginning of the Period							$ 4,874	$ 4,985
Service cost							0	0	$ 1
Interest cost							123	158	198
Plan amendment							0	(20)
Settlements	£ (93)	$ (125)		£ (42)	$ (61)		(496)	(159)
Actuarial loss (gain)							(22)	32
Benefit payments							(146)	(242)
Change in discount rate							122	1,079
Foreign currency impact							438	(959)
Balance at the end of the period		4,893	$ 4,874				4,893	4,874	4,985
Accumulated benefit obligation at end of year		4,893	4,874				4,893	4,874
Change in fair value of plan assets
Balance at the beginning of the period							5,675	5,903
Actual return on plan assets							274	1,233
Employer contributions							86	67
Settlements						£ (371)	(496)	(159)
Benefit payments				£ (116)	$ (159)		(146)	(242)
Foreign currency impact							513	(1,127)
Balance at the end of the period		5,906	5,675				5,906	5,675	5,903
Market related value at end of year		5,906	5,675				5,906	5,675
Funded status		1,013	801				1,013	801
Unrecognized prior-service cost		19	19				19	19
Unrecognized loss		1,217	1,237				1,217	1,237
Net amount recognized		2,249	2,057				2,249	2,057
U.S.
Change in projected benefit obligation
Balance at the beginning of the Period							2,902	3,154
Service cost							0	0	0
Interest cost							96	111	131
Plan amendment							0	0
Settlements			(158)				0	(281)
Actuarial loss (gain)							127	(43)
Benefit payments							(152)	(139)
Change in discount rate							182	100
Foreign currency impact							0	0
Balance at the end of the period		3,155	2,902				3,155	2,902	3,154
Accumulated benefit obligation at end of year		3,155	2,902				3,155	2,902
Change in fair value of plan assets
Balance at the beginning of the period							1,683	1,951
Actual return on plan assets							308	116
Employer contributions							119	36
Settlements							0	(281)
Benefit payments			(281)				(152)	(139)
Foreign currency impact							0	0
Balance at the end of the period		1,958	1,683				1,958	1,683	1,951
Market related value at end of year		1,926	1,819				1,926	1,819
Funded status		(1,197)	(1,219)				(1,197)	(1,219)
Unrecognized prior-service cost		5	6				5	6
Unrecognized loss		1,701	1,612				1,701	1,612
Net amount recognized		509	399				509	399
Other
Change in projected benefit obligation
Balance at the beginning of the Period							1,227	1,177
Service cost							0	0	0
Interest cost							26	29	33
Plan amendment							0	0
Settlements							0	0
Actuarial loss (gain)							16	(7)
Benefit payments							(39)	(39)
Change in discount rate							33	100
Foreign currency impact							138	(33)
Balance at the end of the period		1,401	1,227				1,401	1,227	1,177
Accumulated benefit obligation at end of year		1,373	1,191				1,373	1,191
Change in fair value of plan assets
Balance at the beginning of the period							1,076	1,019
Actual return on plan assets							70	111
Employer contributions							21	20
Settlements							0	0
Benefit payments							(39)	(39)
Foreign currency impact							128	(35)
Balance at the end of the period		1,256	1,076				1,256	1,076	$ 1,019
Market related value at end of year		1,256	1,076				1,256	1,076
Funded status		(145)	(151)				(145)	(151)
Unrecognized prior-service cost		(7)	(6)				(7)	(6)
Unrecognized loss		459	400				459	400
Net amount recognized		$ 307	$ 243				$ 307	$ 243