Pension and other postretirement employee benefit plans - Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension plans, U.S.
Amortization:
Total defined contribution plans	$ 119	$ 115	$ 323	$ 333
Pension plans, Non-U.S.
Amortization:
Total defined contribution plans	85	87	254	254
Pension plans, U.S.
Components of net periodic benefit cost
Benefits earned during the period	85	70	255	210
Interest cost on benefit obligations	125	133	375	401
Expected return on plan assets	(232)	(247)	(697)	(739)
Amortization:
Net (gain)/loss	62	6	185	19
Prior service cost/(credit)	(9)	(9)	(26)	(31)
Net periodic defined benefit cost	31	(47)	92	(140)
Other defined benefit pension plans	3	3	10	10
Total defined benefit plans	34	(44)	102	(130)
Total pension and OPEB cost included in compensation expense	153	71	425	203
Pension plans, Non-U.S.
Components of net periodic benefit cost
Benefits earned during the period	9	8	28	25
Interest cost on benefit obligations	28	34	84	104
Expected return on plan assets	(38)	(42)	(113)	(131)
Amortization:
Net (gain)/loss	9	12	27	36
Prior service cost/(credit)	0	(1)	(1)	(1)
Net periodic defined benefit cost	8	11	25	33
Other defined benefit pension plans	2	2	7	5
Total defined benefit plans	10	13	32	38
Total pension and OPEB cost included in compensation expense	95	100	286	292
OPEB plans
Components of net periodic benefit cost
Benefits earned during the period	0	0	0	0
Interest cost on benefit obligations	8	9	24	27
Expected return on plan assets	(27)	(25)	(80)	(75)
Amortization:
Net (gain)/loss	0	0	0	0
Prior service cost/(credit)	0	0	0	0
Net periodic defined benefit cost	(19)	(16)	(56)	(48)
Total defined benefit plans	(19)	(16)	(56)	(48)
Total pension and OPEB cost included in compensation expense	$ (19)	$ (16)	$ (56)	$ (48)