Financial Derivatives - Summary of Amounts of Gain (Loss) on Interest Rate Derivative Instruments Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Gain (loss) on interest rate derivative instruments recognized in other comprehensive income	$ (13.4)	$ (1.6)	$ 0.0
Designated as Hedging Instrument [Member] \| Interest Rate Swaps [Member]
Derivative [Line Items]
Gain (loss) on interest rate derivative instruments recognized in other comprehensive income	$ (13.4)	$ (1.6)

Amount, after tax and before reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e637-108580

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