Employee benefit plans - Changes in benefit obligations, plan assets and funded status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets and (liabilities) recognized in the Consolidated Balance Sheets:
Pension and other postretirement benefits	$ (196)	$ (218)
Decrease in pension benefit obligation	59
Decrease in fair value of plan assets	49
Pensions Plans
Change in benefit obligation
Benefit obligation at beginning of year	654	614
Service cost	6	5
Interest cost	22	24	$ 26
Actuarial (gains) losses	(43)	38
Benefits paid	(37)	(32)
Foreign exchange	(7)	5
Benefit obligation at end of year	595	654	614
Change in plan assets
Fair value of plan assets at beginning of year	478	437
Actual return on plan assets	(20)	59
Actuarial gains (losses) on plan assets	(32)	(29)	(31)
Employer contributions	16	9
Benefits paid	(35)	(30)
Expenses paid	(2)	(3)	(4)
Expenses paid	(3)	(3)
Foreign exchange	(7)	6
Fair value of plan assets at end of year	429	478	437
Funded status at end of year	(166)	(176)
Assets and (liabilities) recognized in the Consolidated Balance Sheets:
Other assets	4	4
Pension and other postretirement benefits	(170)	(180)
Total	(166)	(176)
Postretirement Benefit Plans
Change in benefit obligation
Benefit obligation at beginning of year	39	64
Valuation effect at beginning of year		(2)
Interest cost	2	2	2
Actuarial (gains) losses	(4)	1
Prior service credit		(1)
Benefits paid	(2)	(2)
Curtailments (1)		(24)
Foreign exchange	(3)	1
Benefit obligation at end of year	32	39	$ 64
Change in plan assets
Employer contributions	3	2
Benefits paid	(3)	(2)
Funded status at end of year	(32)	(39)
Assets and (liabilities) recognized in the Consolidated Balance Sheets:
Accrued compensation and benefits	(1)	(2)
Pension and other postretirement benefits	(31)	(37)
Total	$ (32)	$ (39)