Employee Benefits - Components of Net Periodic (Benefit) Cost (Details) £ in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 GBP (£)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
U.K.
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0		$ 0	$ 0	$ 0
Interest cost	21		27	43	55
Expected return on plan assets, net of administration expenses	(39)		(48)	(78)	(97)
Amortization of prior-service cost	1		0	1	1
Amortization of net actuarial loss	8		8	15	15
Net periodic (benefit) cost	(9)		(13)	(19)	(26)
Loss on pension settlement	2	£ 2	0	2	0
Total net periodic (benefit) cost	(7)		(13)	(17)	(26)
Other
Defined Benefit Plan Disclosure [Line Items]
Service cost	0		0	0	0
Interest cost	5		7	9	14
Expected return on plan assets, net of administration expenses	(9)		(10)	(17)	(20)
Amortization of prior-service cost	0		0	0	0
Amortization of net actuarial loss	3		3	6	6
Net periodic (benefit) cost	(1)			(2)
Loss on pension settlement	0		0	0	0
Total net periodic (benefit) cost	(1)		0	(2)	0
U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	0		0	0	0
Interest cost	21		27	42	54
Expected return on plan assets, net of administration expenses	(33)		(34)	(66)	(68)
Amortization of prior-service cost	0		0	0	1
Amortization of net actuarial loss	17		14	34	27
Net periodic (benefit) cost	5		7	10	14
Loss on pension settlement	0		0	0	0
Total net periodic (benefit) cost	$ 5		$ 7	$ 10	$ 14