Employee Benefits (Details) £ in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 GBP (£)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 GBP (£)	Jun. 30, 2018 USD ($)
U.K.
Defined Benefit Plan Disclosure
Service cost	$ 0		$ 0	$ 0		$ 0
Interest cost	27		29	55		58
Expected return on plan assets, net of administration expenses	(48)		(50)	(97)		(101)
Amortization of prior-service cost	0		0	1		0
Amortization of net actuarial loss	8		7	15		15
Net periodic (benefit) cost	(13)		(14)	(26)		(28)
Loss on pension settlement	0		16	0		23
Total net periodic (benefit) cost	(13)		2	(26)		(5)
Payment for settlement					£ 99	132
Benefit obligation period increase (decrease)		£ (87)	(115)
Benefit obligation, (increase) decrease for settlement		£ 12	16		£ 17	23
Estimate of contributions to defined benefit pension plans for the current fiscal year	80			80
Contributions made to defined benefit pension plans	23		25	46		48
U.S.
Defined Benefit Plan Disclosure
Service cost	0		0	0		0
Interest cost	27		25	54		50
Expected return on plan assets, net of administration expenses	(34)		(36)	(68)		(72)
Amortization of prior-service cost	0		1	1		1
Amortization of net actuarial loss	14		15	27		30
Net periodic (benefit) cost	7		5	14		9
Loss on pension settlement	0		0	0		0
Total net periodic (benefit) cost	7		5	14		9
Estimate of contributions to defined benefit pension plans for the current fiscal year	46			46
Contributions made to defined benefit pension plans	6		8	23		25
Other
Defined Benefit Plan Disclosure
Service cost	0		0	0		0
Interest cost	7		7	14		14
Expected return on plan assets, net of administration expenses	(10)		(11)	(20)		(23)
Amortization of prior-service cost	0		0	0		0
Amortization of net actuarial loss	3		3	6		6
Net periodic (benefit) cost	0		(1)	0		(3)
Loss on pension settlement	0		0	0		0
Total net periodic (benefit) cost	0		(1)	0		(3)
Estimate of contributions to defined benefit pension plans for the current fiscal year	19			19
Contributions made to defined benefit pension plans	$ 3		$ 3	$ 10		$ 11