Employee benefit plans - Narrative (Details) - Pension $ in Millions	3 Months Ended
Mar. 31, 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.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration