Employee benefit plans - Changes in benefit obligations, plan assets and funded status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets and (liabilities) recognized in the Consolidated Balance Sheets:
Pension and other postretirement benefits	$ (197)	$ (178)
Decrease in pension benefit obligation	64
Decrease in fair value of plan assets	52
Aggregate accumulated benefit obligation	583	519
Pension
Change in benefit obligation
Benefit obligation at beginning of year	519	568
Service cost	2	2
Interest cost	21	19	$ 21
Actuarial (gains) losses	71	(37)
Benefits paid	(30)	(33)
Benefit obligation at end of year	583	519	568
Change in plan assets
Fair value of plan assets at beginning of year	352	391
Actuarial gains (losses) on plan assets	70	(19)
Employer contributions	12	13
Benefits paid	(28)	(31)
Expenses paid	(2)	(2)	(2)
Expenses paid	(2)	(2)
Fair value of plan assets at end of year	404	352	391
Funded status at end of year	(179)	(167)
Assets and (liabilities) recognized in the Consolidated Balance Sheets:
Accrued compensation and benefits	(1)	(1)
Pension and other postretirement benefits	(178)	(166)
Total	(179)	(167)
Other Postretirement
Change in benefit obligation
Benefit obligation at beginning of year	13	16
Interest cost	1	1	2
Actuarial (gains) losses		(2)
Benefits paid		(2)
Benefit obligation at end of year	14	13	$ 16
Change in plan assets
Employer contributions		2
Benefits paid		(2)
Funded status at end of year	(14)	(13)
Assets and (liabilities) recognized in the Consolidated Balance Sheets:
Accrued compensation and benefits	(1)	(1)
Pension and other postretirement benefits	(13)	(12)
Total	$ (14)	$ (13)