Goodwill and intangible assets - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2024	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Indefinite-lived intangible assets	$ 763.0	$ 762.8
Cumulative translation adjustment	$ 5.4

Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482573/350-20-50-1

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