Fair value measurements - Level 3 (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Changes in level 3
Fair value adjustments to long lived assets	$ 0	$ 0
Non-recurring
Changes in level 3
Fair value measurements	$ 0	$ 0

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration