Employee Benefits - Components of Net Periodic (Benefit) Cost (Details) £ in Millions, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 GBP (£)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)
U.K.
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0		$ 0	$ 0
Interest cost	16	22		49	65
Expected return on plan assets, net of administration expenses	(34)	(40)		(103)	(118)
Amortization of prior-service cost	1	0		3	1
Amortization of net actuarial loss	8	8		24	23
Net periodic (benefit) cost				(27)	(29)
Loss on pension settlement			£ 2	0	2
Total net periodic (benefit) cost	(9)	(10)		(27)	(27)
Other
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0		0	0
Interest cost	3	5		9	14
Expected return on plan assets, net of administration expenses	(8)	(8)		(24)	(25)
Amortization of prior-service cost	0	0		0	0
Amortization of net actuarial loss	4	3		11	9
Net periodic (benefit) cost				(4)	(2)
Loss on pension settlement				0	0
Total net periodic (benefit) cost	(1)	0		(4)	(2)
U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0		0	0
Interest cost	15	22		43	64
Expected return on plan assets, net of administration expenses	(33)	(34)		(97)	(100)
Amortization of prior-service cost	0	0		0	0
Amortization of net actuarial loss	19	17		58	51
Net periodic (benefit) cost				4	15
Loss on pension settlement				0	0
Total net periodic (benefit) cost	$ 1	$ 5		$ 4	$ 15