Employee Benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
U.K. [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1	$ 1	$ 1
Interest cost	198	230	210
Expected return on plan assets, net of administration expenses	(307)	(326)	(302)
Amortization of prior-service cost	1	1	1
Amortization of net actuarial loss	41	52	49
Curtailment gain and other	0	0	0
Net periodic benefit (income) cost	(66)	(42)	(41)
U.S. [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	2	7
Interest cost	131	129	114
Expected return on plan assets, net of administration expenses	(154)	(157)	(139)
Amortization of prior-service cost	2	2	0
Amortization of net actuarial loss	54	42	52
Curtailment gain and other	0	0	0
Net periodic benefit (income) cost	33	18	34
Other [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	18
Interest cost	33	47	45
Expected return on plan assets, net of administration expenses	(50)	(59)	(59)
Amortization of prior-service cost	0	0	0
Amortization of net actuarial loss	11	10	23
Curtailment gain and other	0	(2)	0
Net periodic benefit (income) cost	$ (6)	$ (4)	$ 27