Postretirement benefit plans - Benefit obligations and plan assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Funded status (FVPA – BO) at end of year	$ (113)	$ (98)
U.S. defined benefit plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	955	1,098
Service cost	21	26	24
Interest cost	45	45	44
Participant contributions	0	0
Benefits paid	(66)	(9)
Medicare subsidy	0	0
Actuarial loss (gain)	133	(27)
Settlements	(12)	(178)
Curtailments	0	0
Effects of exchange rate changes	0	0
Other	0	0
Benefit obligation at end of year (BO)	1,076	955	1,098
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	941	1,071
Actual return on plan assets	132	1
Employer contributions (funding of qualified plans)	75	43
Employer contributions (payments for non-qualified plans)	12	13
Participant contributions	0	0
Benefits paid	(66)	(9)
Medicare subsidy	0	0
Settlements	(12)	(178)
Effects of exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year (FVPA)	1,082	941	1,071
Funded status (FVPA – BO) at end of year	6	(14)
U.S. retiree health care plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	472	509
Service cost	4	5	5
Interest cost	22	20	25
Participant contributions	19	18
Benefits paid	(45)	(47)
Medicare subsidy	4	3
Actuarial loss (gain)	37	(36)
Settlements	0	0
Curtailments	0	0
Effects of exchange rate changes	0	0
Other	0	0
Benefit obligation at end of year (BO)	513	472	509
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	485	517
Actual return on plan assets	24	41
Employer contributions (funding of qualified plans)	10	0
Employer contributions (payments for non-qualified plans)	0	0
Participant contributions	19	18
Benefits paid	(45)	(45)
Medicare subsidy	4	0
Settlements	0	0
Effects of exchange rate changes	0	0
Other	0	(46)
Fair value of plan assets at end of year (FVPA)	497	485	517
Funded status (FVPA – BO) at end of year	(16)	13
Non-U.S. defined benefit plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	2,276	2,414
Service cost	39	41	45
Interest cost	68	61	75
Participant contributions	5	1
Benefits paid	(84)	(81)
Medicare subsidy	0	0
Actuarial loss (gain)	275	96
Settlements	(7)	(30)
Curtailments	(11)	(28)
Effects of exchange rate changes	(245)	(237)
Other	0	39
Benefit obligation at end of year (BO)	2,316	2,276	2,414
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2,179	2,218
Actual return on plan assets	295	201
Employer contributions (funding of qualified plans)	64	62
Employer contributions (payments for non-qualified plans)	0	0
Participant contributions	5	1
Benefits paid	(84)	(81)
Medicare subsidy	0	0
Settlements	(7)	(30)
Effects of exchange rate changes	(239)	(232)
Other	0	40
Fair value of plan assets at end of year (FVPA)	2,213	2,179	2,218
Funded status (FVPA – BO) at end of year	$ (103)	$ (97)

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including perscription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations. Payments are made from plan assets, resulting in a decrease to plan assets, and from company assets.

No definition available.