Employee benefit plan - Narrative (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Plan assets	$ 0	$ 0

Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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Name:	us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant