Postretirement benefit plans - Plan assets by level three heirarchy (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
U.S. defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,082	$ 941	$ 1,071
U.S. defined benefit plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,082	904
U.S. defined benefit plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	37	37
U.S. defined benefit plan | Fixed income securities and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	707	607
U.S. defined benefit plan | Fixed income securities and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Fixed income securities and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	707	607
U.S. defined benefit plan | Fixed income securities and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	375	297
U.S. defined benefit plan | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	375	297
U.S. defined benefit plan | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	37
U.S. defined benefit plan | Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. defined benefit plan | Other | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	37
U.S. retiree health care plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	497	485	517
U.S. retiree health care plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	200	193
U.S. retiree health care plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297	292
U.S. retiree health care plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. retiree health care plan | Fixed income securities and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	243	238
U.S. retiree health care plan | Fixed income securities and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	200	193
U.S. retiree health care plan | Fixed income securities and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43	45
U.S. retiree health care plan | Fixed income securities and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. retiree health care plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	254	247
U.S. retiree health care plan | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. retiree health care plan | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	254	247
U.S. retiree health care plan | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,213	2,179	2,218
Non-U.S. defined benefit plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	436	403
Non-U.S. defined benefit plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,772	1,768
Non-U.S. defined benefit plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	8	19
Non-U.S. defined benefit plan | Fixed income securities and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,608	1,521
Non-U.S. defined benefit plan | Fixed income securities and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	430	397
Non-U.S. defined benefit plan | Fixed income securities and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,178	1,124
Non-U.S. defined benefit plan | Fixed income securities and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. defined benefit plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	600	650
Non-U.S. defined benefit plan | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	6
Non-U.S. defined benefit plan | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	594	644
Non-U.S. defined benefit plan | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. defined benefit plan | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	8
Non-U.S. defined benefit plan | Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. defined benefit plan | Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. defined benefit plan | Other | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 5	$ 8