Postemployment benefit plans (Details 4) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.S. Pension Benefits
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	$ 13,416	$ 11,354	$ 11,440
U.S. Pension Benefits | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	2,930	2,995
U.S. Pension Benefits | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,588	1,658
U.S. Pension Benefits | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	5,995	4,389
U.S. Pension Benefits | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,165	710
U.S. Pension Benefits | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	793	647
U.S. Pension Benefits | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	369	491
U.S. Pension Benefits | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	68	90
U.S. Pension Benefits | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	10	10
U.S. Pension Benefits | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	498	364
U.S. Pension Benefits | Level 1
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	4,491	4,562
U.S. Pension Benefits | Level 1 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	2,745	2,816
U.S. Pension Benefits | Level 1 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,573	1,653
U.S. Pension Benefits | Level 1 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 1 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 1 | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 1 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 1 | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 1 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	173	93
U.S. Pension Benefits | Level 2
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	8,397	6,290
U.S. Pension Benefits | Level 2 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 2 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	15	5
U.S. Pension Benefits | Level 2 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	5,886	4,273
U.S. Pension Benefits | Level 2 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,165	710
U.S. Pension Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	793	647
U.S. Pension Benefits | Level 2 | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	369	491
U.S. Pension Benefits | Level 2 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	68	90
U.S. Pension Benefits | Level 2 | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 2 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	101	74
U.S. Pension Benefits | Level 3
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	90	57
U.S. Pension Benefits | Level 3 | Equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	20	5	5
U.S. Pension Benefits | Level 3 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	20	5
U.S. Pension Benefits | Level 3 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 3 | Fixed income securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	60	31	42
U.S. Pension Benefits | Level 3 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	60	31
U.S. Pension Benefits | Level 3 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 3 | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 3 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Level 3 | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	10	10	9
U.S. Pension Benefits | Level 3 | Other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	11	3
U.S. Pension Benefits | Level 3 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	11
U.S. Pension Benefits | Measured at NAV
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	438	445
U.S. Pension Benefits | Measured at NAV | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	165	174
U.S. Pension Benefits | Measured at NAV | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Measured at NAV | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	49	85
U.S. Pension Benefits | Measured at NAV | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Measured at NAV | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Measured at NAV | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Measured at NAV | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Measured at NAV | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
U.S. Pension Benefits | Measured at NAV | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	224	186
Non-U.S. Pension Benefits
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	4,305	3,887	3,890
Non-U.S. Pension Benefits | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	192	513
Non-U.S. Pension Benefits | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,363	796
Non-U.S. Pension Benefits | Global equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	149	156
Non-U.S. Pension Benefits | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	156	152
Non-U.S. Pension Benefits | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	374	356
Non-U.S. Pension Benefits | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	64	44
Non-U.S. Pension Benefits | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,229	1,172
Non-U.S. Pension Benefits | Global fixed income
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	468	420
Non-U.S. Pension Benefits | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	186	179
Non-U.S. Pension Benefits | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	124	99
Non-U.S. Pension Benefits | Level 1
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	635	1,219
Non-U.S. Pension Benefits | Level 1 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	55	405
Non-U.S. Pension Benefits | Level 1 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	400	639
Non-U.S. Pension Benefits | Level 1 | Global equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	116	122
Non-U.S. Pension Benefits | Level 1 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 1 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 1 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 1 | Global fixed income
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 1 | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 1 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	64	53
Non-U.S. Pension Benefits | Level 2
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	2,375	2,230
Non-U.S. Pension Benefits | Level 2 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 2 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	34	38
Non-U.S. Pension Benefits | Level 2 | Global equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	33	34
Non-U.S. Pension Benefits | Level 2 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	156	151
Non-U.S. Pension Benefits | Level 2 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	363	345
Non-U.S. Pension Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	64	44
Non-U.S. Pension Benefits | Level 2 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,229	1,172
Non-U.S. Pension Benefits | Level 2 | Global fixed income
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	250	221
Non-U.S. Pension Benefits | Level 2 | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	186	179
Non-U.S. Pension Benefits | Level 2 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	60	46
Non-U.S. Pension Benefits | Level 3
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	5	2
Non-U.S. Pension Benefits | Level 3 | Equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0	2
Non-U.S. Pension Benefits | Level 3 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 3 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 3 | Global equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 3 | Fixed income securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	5	2	5
Non-U.S. Pension Benefits | Level 3 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	1
Non-U.S. Pension Benefits | Level 3 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	5	1
Non-U.S. Pension Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 3 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 3 | Global fixed income
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Level 3 | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0	0
Non-U.S. Pension Benefits | Level 3 | Other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0	0
Non-U.S. Pension Benefits | Level 3 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Measured at NAV
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1,290	436
Non-U.S. Pension Benefits | Measured at NAV | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	137	108
Non-U.S. Pension Benefits | Measured at NAV | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	929	119
Non-U.S. Pension Benefits | Measured at NAV | Global equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Measured at NAV | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Measured at NAV | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	6	10
Non-U.S. Pension Benefits | Measured at NAV | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Measured at NAV | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Measured at NAV | Global fixed income
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	218	199
Non-U.S. Pension Benefits | Measured at NAV | Real estate
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Non-U.S. Pension Benefits | Measured at NAV | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	504	550	$ 650
Other Postretirement Benefits | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	256	277
Other Postretirement Benefits | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	103	110
Other Postretirement Benefits | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	61	70
Other Postretirement Benefits | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	15	17
Other Postretirement Benefits | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	17	16
Other Postretirement Benefits | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	34	31
Other Postretirement Benefits | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	4	4
Other Postretirement Benefits | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	14	25
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	358	391
Other Postretirement Benefits | Level 1 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	255	276
Other Postretirement Benefits | Level 1 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	103	110
Other Postretirement Benefits | Level 1 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 1 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 1 | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 1 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 1 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	5
Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	134	139
Other Postretirement Benefits | Level 2 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1	1
Other Postretirement Benefits | Level 2 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 2 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	60	69
Other Postretirement Benefits | Level 2 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	15	17
Other Postretirement Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	17	16
Other Postretirement Benefits | Level 2 | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	34	31
Other Postretirement Benefits | Level 2 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	4	4
Other Postretirement Benefits | Level 2 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	3	1
Other Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Level 3 | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	12	20
Other Postretirement Benefits | Measured at NAV | U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV | Non-U.S. equities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV | U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	1	1
Other Postretirement Benefits | Measured at NAV | Non-U.S. corporate bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV | U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV | U.S. governmental agency mortgage-backed securities
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV | Non-U.S. government bonds
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	0	0
Other Postretirement Benefits | Measured at NAV | Cash, short-term instruments and other
Defined Benefit Plan Disclosure
Total Assets, at Fair Value	$ 11	$ 19