Income taxes - Additional Information (Details) $ in Millions	Sep. 30, 2018 USD ($)
Income Tax Disclosure [Abstract]
Provisional transition tax liability	$ 22.7

Amount of reasonable estimate for tax liability for which accounting for tax effect from transition tax on accumulated earnings of controlled foreign corporation deemed repatriated is incomplete pursuant to Tax Cuts and Jobs Act.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 2A
-Subparagraph (SAB Topic 5.EE.Q2(b))
-URI http://asc.fasb.org/extlink&oid=116825942&loc=SL116722634-122817

Name:	us-gaap_TaxCutsAndJobsActOf2017IncompleteAccountingTransitionTaxForAccumulatedForeignEarningsProvisionalLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant