Employee Benefits - Components of Net Periodic (Benefit) Cost (Details) £ in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 GBP (£)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)
U.K.
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0		$ 0	$ 0
Interest cost	17	21		33	43
Expected return on plan assets, net of administration expenses	(35)	(39)		(69)	(78)
Amortization of prior-service cost	1	1		2	1
Amortization of net actuarial loss	8	8		16	15
Net periodic (benefit) cost	(9)	(9)		(18)	(19)
Loss on pension settlement	0	2	£ 2	0	2
Total net periodic (benefit) cost	(9)	(7)		(18)	(17)
Other
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0		0	0
Interest cost	3	5		6	9
Expected return on plan assets, net of administration expenses	(8)	(9)		(16)	(17)
Amortization of prior-service cost	0	0		0	0
Amortization of net actuarial loss	3	3		7	6
Net periodic (benefit) cost	(2)	(1)		(3)	(2)
Loss on pension settlement	0	0		0	0
Total net periodic (benefit) cost	(2)	(1)		(3)	(2)
U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0		0	0
Interest cost	14	21		28	42
Expected return on plan assets, net of administration expenses	(32)	(33)		(64)	(66)
Amortization of prior-service cost	0	0		0	0
Amortization of net actuarial loss	20	17		39	34
Net periodic (benefit) cost	2	5		3	10
Loss on pension settlement	0	0		0	0
Total net periodic (benefit) cost	$ 2	$ 5		$ 3	$ 10