Employee Benefits (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 USD ($)
U.K.
Defined Benefit Plan Disclosure
Interest cost	$ 28		$ 29
Expected return on plan assets, net of administration expenses	(49)		(51)
Amortization of prior-service cost	1		0
Amortization of net actuarial loss	7		8
Net periodic (benefit) cost	(13)		(14)
Loss on pension settlement	0		7
Total net periodic (benefit) cost	(13)		(7)
Payment for settlement		£ 48	68
Benefit obligation period increase (decrease)		(44)	(63)
Benefit obligation, (increase) decrease for settlement		£ 5	7
Estimate of contributions to defined benefit pension plans for the current fiscal year	80
Contributions made to defined benefit pension plans	23		23
U.S.
Defined Benefit Plan Disclosure
Interest cost	27		25
Expected return on plan assets, net of administration expenses	(34)		(36)
Amortization of prior-service cost	1		0
Amortization of net actuarial loss	13		15
Net periodic (benefit) cost	7		4
Loss on pension settlement	0		0
Total net periodic (benefit) cost	7		4
Estimate of contributions to defined benefit pension plans for the current fiscal year	46
Contributions made to defined benefit pension plans	17		17
Other
Defined Benefit Plan Disclosure
Interest cost	7		7
Expected return on plan assets, net of administration expenses	(10)		(12)
Amortization of prior-service cost	0		0
Amortization of net actuarial loss	3		3
Net periodic (benefit) cost	0		(2)
Loss on pension settlement	0		0
Total net periodic (benefit) cost	0		(2)
Estimate of contributions to defined benefit pension plans for the current fiscal year	19
Contributions made to defined benefit pension plans	$ 7		$ 8