Employee Benefits - Components of net periodic benefit cost (Details) - Pension Plan £ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost				$ 0	$ 0	$ 0
Interest cost				109	123	158
Expected return on plan assets, net of administration expenses				(192)	(199)	(243)
Amortization of prior-service cost				1	1	2
Amortization of net actuarial loss				28	31	31
Net periodic benefit (income) cost				(54)	(44)	(52)
Settlement expense	£ (93)	$ 125	£ (28)	37	125	61
Total net periodic benefit cost (income)				(17)	81	9
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost				0	0	0
Interest cost				99	96	111
Expected return on plan assets, net of administration expenses				(144)	(140)	(156)
Amortization of prior-service cost				2	2	2
Amortization of net actuarial loss				59	50	50
Net periodic benefit (income) cost				16	8	7
Settlement expense				0	0	158
Total net periodic benefit cost (income)				16	8	165
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost				0	0	0
Interest cost				27	26	29
Expected return on plan assets, net of administration expenses				(45)	(47)	(48)
Amortization of prior-service cost				0	0	0
Amortization of net actuarial loss				12	11	10
Net periodic benefit (income) cost				(6)	(10)	(9)
Settlement expense				0	0	0
Total net periodic benefit cost (income)				$ (6)	$ (10)	$ (9)