Employee Benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0	$ 1	$ 1
Interest cost	158	198	230
Expected return on plan assets, net of administration expenses	(243)	(307)	(326)
Amortization of prior-service cost	2	1	1
Amortization of net actuarial loss	31	41	52
Net periodic benefit (income) cost	(52)	(66)	(42)
Settlement expense	61	0	0
Curtailment gain and other	0	0	0
Total net periodic benefit cost (income)	9	(66)	(42)
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	2
Interest cost	111	131	129
Expected return on plan assets, net of administration expenses	(156)	(154)	(157)
Amortization of prior-service cost	2	2	2
Amortization of net actuarial loss	50	54	42
Net periodic benefit (income) cost	7	33	18
Settlement expense	158	0	0
Curtailment gain and other	0	0	0
Total net periodic benefit cost (income)	165	33	18
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	0
Interest cost	29	33	47
Expected return on plan assets, net of administration expenses	(48)	(50)	(59)
Amortization of prior-service cost	0	0	0
Amortization of net actuarial loss	10	11	10
Net periodic benefit (income) cost	(9)	(6)	(2)
Settlement expense	0	0
Curtailment gain and other	0	0	(2)
Total net periodic benefit cost (income)	$ (9)	$ (6)	$ (4)