Employee Benefits - Components of Net Periodic (Benefit) Cost (Details) £ in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 GBP (£)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
U.K.
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0			$ 0	$ 0	$ 0
Interest cost	22			26	65	81
Expected return on plan assets, net of administration expenses	(40)			(46)	(118)	(143)
Amortization of prior-service cost	0			0	1	1
Amortization of net actuarial loss	8			7	23	22
Net periodic (benefit) cost					(29)	(39)
Loss on pension settlement		$ 2	£ 2		2	0
Total net periodic (benefit) cost	(10)			(13)	(27)	(39)
Other
Defined Benefit Plan Disclosure [Line Items]
Service cost	0			0	0	0
Interest cost	5			6	14	20
Expected return on plan assets, net of administration expenses	(8)			(9)	(25)	(29)
Amortization of prior-service cost	0			0	0	0
Amortization of net actuarial loss	3			3	9	9
Net periodic (benefit) cost					(2)
Loss on pension settlement					0	0
Total net periodic (benefit) cost	0			0	(2)	0
U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	0			0	0	0
Interest cost	22			27	64	81
Expected return on plan assets, net of administration expenses	(34)			(34)	(100)	(102)
Amortization of prior-service cost	0			1	0	2
Amortization of net actuarial loss	17			13	51	40
Net periodic (benefit) cost					15	21
Loss on pension settlement					0	0
Total net periodic (benefit) cost	$ 5			$ 7	$ 15	$ 21