Employee Benefits - Components of net periodic benefit cost (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0	$ 0	$ 1
Interest cost	123	158	198
Expected return on plan assets, net of administration expenses	(199)	(243)	(307)
Amortization of prior-service cost	1	2	1
Amortization of net actuarial loss	31	31	41
Net periodic benefit (income) cost	(44)	(52)	(66)
Settlement expense	125	61	0
Total net periodic benefit cost (income)	81	9	(66)
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	0
Interest cost	96	111	131
Expected return on plan assets, net of administration expenses	(140)	(156)	(154)
Amortization of prior-service cost	2	2	2
Amortization of net actuarial loss	50	50	54
Net periodic benefit (income) cost	8	7	33
Settlement expense	0	158	0
Total net periodic benefit cost (income)	8	165	33
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	0
Interest cost	26	29	33
Expected return on plan assets, net of administration expenses	(47)	(48)	(50)
Amortization of prior-service cost	0	0	0
Amortization of net actuarial loss	11	10	11
Net periodic benefit (income) cost	(10)	(9)	(6)
Settlement expense	0	0	0
Total net periodic benefit cost (income)	$ (10)	$ (9)	$ (6)