Pension and other postretirement employee benefit plans (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
U S Defined Benefit Pension And OPEB Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 16,600	$ 16,500
U.S. non-qualified defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, non-U.S., expected contribution	33
Pension plans, Non-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,700	$ 3,700
Defined benefit pension plan, non-U.S., expected contribution	47
OPEB plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, non-U.S., expected contribution	$ 2