Employee Benefits - Components of net periodic benefit cost (Details) - Pension Plan £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0		$ 0		$ 0
Interest cost	85		108		99
Expected return on plan assets, net of administration expenses	(134)		(136)		(144)
Amortization of prior-service cost	1		2		2
Amortization of net actuarial loss	68		53		59
Net periodic benefit (income) cost	20		27		16
Settlement expense	0		0		0
Total net periodic benefit cost (income)	20		27		16
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0		0		0
Interest cost	88		109		109
Expected return on plan assets, net of administration expenses	(159)		(191)		(192)
Amortization of prior-service cost	2		1		1
Amortization of net actuarial loss	30		29		28
Net periodic benefit (income) cost	(39)		(52)		(54)
Settlement expense	2	£ 2	5	£ 4	37	£ 28
Total net periodic benefit cost (income)	(37)		(47)		(17)
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0		0		0
Interest cost	19		27		27
Expected return on plan assets, net of administration expenses	(34)		(40)		(45)
Amortization of prior-service cost	0		0		0
Amortization of net actuarial loss	12		12		12
Net periodic benefit (income) cost	(3)		(1)		(6)
Settlement expense	0		0		0
Total net periodic benefit cost (income)	$ (3)		$ (1)		$ (6)