Employee benefit plans - Narrative (Details) - Pension $ in Millions	6 Months Ended
Jun. 30, 2020 USD ($)
Employee benefit plans
Employer contributions	$ 3
Expected future contribution by the employer	$ 15

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration