Subsequent events - Narrative (Details) - IAM National Pension Fund - Subsequent event $ in Thousands	Nov. 26, 2019 USD ($) union
Subsequent Event [Line Items]
Number of unions \| union	1
Withdrawal liability \| $	$ 745
Payment period	20 years

Amount of the obligation recognized by withdrawing from a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

No definition available.

Name:	us-gaap_MultiemployerPlansWithdrawalObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant