Subsequent events (Details) $ in Millions	1 Months Ended
Oct. 31, 2018 USD ($)
Subsequent events
Subsequent events
Business interruption claims received for Hurricane Irma and Maria	$ 5.5

Amount of insurance proceeds for an event or transaction that is unusual in nature or infrequent in occurrence, or both.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=114868883&loc=SL114871943-224233

Name:	us-gaap_UnusualOrInfrequentItemInsuranceProceeds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration