Employee Benefits - Components of net periodic benefit cost (Details) - Pension Plan £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0		$ 0		$ 0
Interest cost	57		85		108
Expected return on plan assets, net of administration expenses	(130)		(134)		(136)
Amortization of prior-service cost	0		1		2
Amortization of net actuarial loss	78		68		53
Net periodic benefit (income) cost	5		20		27
Settlement expense	0		0		0
Total net periodic benefit cost (income)	5		20		27
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1		0		0
Interest cost	65		88		109
Expected return on plan assets, net of administration expenses	(137)		(159)		(191)
Amortization of prior-service cost	2		2		1
Amortization of net actuarial loss	32		30		29
Net periodic benefit (income) cost	(37)		(39)		(52)
Settlement expense	5	£ 3	2	£ 2	5	£ 4
Total net periodic benefit cost (income)	(32)		(37)		(47)
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0		0		0
Interest cost	12		19		27
Expected return on plan assets, net of administration expenses	(32)		(34)		(40)
Amortization of prior-service cost	0		0		0
Amortization of net actuarial loss	15		12		12
Net periodic benefit (income) cost	(5)		(3)		(1)
Settlement expense	0		0		0
Total net periodic benefit cost (income)	$ (5)		$ (3)		$ (1)