Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
U.K.
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	31	37	91	123
Expected return on plan assets, net of administration expenses	(50)	(58)	(147)	(187)
Amortization of prior-service cost	0	0	0	1
Amortization of net actuarial loss	8	7	23	24
Net periodic cost (benefit)	(11)	(14)	(33)	(39)
Loss on pension settlement	0	0	0	61
Total net periodic cost (benefit)	(11)	(14)	(33)	22
Estimate of contributions to defined benefit pension plans for the current fiscal year	80		80
Contributions made to defined benefit pension plans	22	19	64	53
U.S.
Defined Benefit Plan Disclosure
Service cost	0	0	0	0
Interest cost	24	28	72	83
Expected return on plan assets, net of administration expenses	(34)	(39)	(104)	(117)
Amortization of prior-service cost	0	1	1	2
Amortization of net actuarial loss	13	12	38	37
Net periodic cost (benefit)	3	2	7	5
Loss on pension settlement	0	0	0	0
Total net periodic cost (benefit)	3	2	7	5
Estimate of contributions to defined benefit pension plans for the current fiscal year	51		51
Non-cash contributions by employer	80		80
Contributions made to defined benefit pension plans	5	5	31	24
Other International Plan [Member]
Defined Benefit Plan Disclosure
Service cost	0	0	0	0
Interest cost	7	7	19	21
Expected return on plan assets, net of administration expenses	(13)	(12)	(35)	(36)
Amortization of prior-service cost	0	0	0	0
Amortization of net actuarial loss	3	3	9	8
Net periodic cost (benefit)	(3)	(2)	(7)	(7)
Loss on pension settlement	0	0	0	0
Total net periodic cost (benefit)	(3)	(2)	(7)	(7)
Estimate of contributions to defined benefit pension plans for the current fiscal year	18		18
Contributions made to defined benefit pension plans	$ 3	$ 4	$ 14	$ 14