Postretirement benefit plans - Expenses related to defined benefit and retiree health care benefit plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
U.S. defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6	$ 5
Interest cost	10	11
Expected return on plan assets	(12)	(10)
Recognized net actuarial loss	5	6
Amortization of prior service cost (credit)	0	0
Net periodic benefit costs	9	12
Settlement losses	1	1
Total, including other postretirement losses	10	13
U.S. retiree health care plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	5	5
Expected return on plan assets	(5)	(5)
Recognized net actuarial loss	2	2
Amortization of prior service cost (credit)	1	1
Net periodic benefit costs	4	4
Settlement losses	0	0
Total, including other postretirement losses	4	4
Non-U.S. defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	9	10
Interest cost	13	17
Expected return on plan assets	(19)	(19)
Recognized net actuarial loss	6	6
Amortization of prior service cost (credit)	0	(1)
Net periodic benefit costs	9	13
Settlement losses	0	0
Total, including other postretirement losses	$ 9	$ 13