Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
U.K.
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0
Interest cost	30	43
Expected return on plan assets, net of administration expenses	(48)	(64)
Amortization of prior-service cost	0	1
Amortization of net actuarial loss	7	8
Net periodic (benefit) cost	(11)	(12)
Estimate of contributions to defined benefit pension plans for the current fiscal year	80
Contributions made to defined benefit pension plans	16	17
U.S.
Defined Benefit Plan Disclosure
Service cost	0	0
Interest cost	24	28
Expected return on plan assets, net of administration expenses	(35)	(39)
Amortization of prior-service cost	0	0
Amortization of net actuarial loss	13	13
Net periodic (benefit) cost	2	2
Estimate of contributions to defined benefit pension plans for the current fiscal year	87
Contributions made to defined benefit pension plans	13	13
Other
Defined Benefit Plan Disclosure
Service cost	0	0
Interest cost	6	7
Expected return on plan assets, net of administration expenses	(11)	(12)
Amortization of prior-service cost	0	0
Amortization of net actuarial loss	3	3
Net periodic (benefit) cost	(2)	(2)
Estimate of contributions to defined benefit pension plans for the current fiscal year	18
Contributions made to defined benefit pension plans	$ 2	$ 7