Employee Benefits (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
U.K.
Defined Benefit Plan Disclosure
Service cost		$ 0	$ 0
Interest cost		29	30
Expected return on plan assets, net of administration expenses		(51)	(48)
Amortization of net actuarial loss		8	7
Net periodic cost (benefit)		(14)	(11)
Loss on pension settlement		7	0
Total net periodic cost (benefit)		(7)	(11)
Estimate of contributions to defined benefit pension plans for the current fiscal year		92
Contributions made to defined benefit pension plans		23	16
U.S.
Defined Benefit Plan Disclosure
Service cost		0	0
Interest cost		25	24
Expected return on plan assets, net of administration expenses		(36)	(35)
Amortization of net actuarial loss		15	13
Net periodic cost (benefit)		4	2
Loss on pension settlement		0	0
Total net periodic cost (benefit)		4	2
Estimate of contributions to defined benefit pension plans for the current fiscal year		63
Contributions made to defined benefit pension plans		17	13
Other International Plan
Defined Benefit Plan Disclosure
Service cost		0	0
Interest cost		7	6
Expected return on plan assets, net of administration expenses		(12)	(11)
Amortization of net actuarial loss		3	3
Net periodic cost (benefit)		(2)	(2)
Loss on pension settlement		0	0
Total net periodic cost (benefit)		(2)	(2)
Estimate of contributions to defined benefit pension plans for the current fiscal year		22
Contributions made to defined benefit pension plans		8	$ 2
Pension Plan | U.K.
Defined Benefit Plan Disclosure
Payment for settlement	£ 48	68
Benefit obligation period increase (decrease)	(44)	(63)
Benefit obligation, (increase) decrease for settlement	£ 5	$ 7