Financial Derivatives - Summary of Amounts of Gain (Loss) on Interest Rate Derivative Instruments Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Gain (loss) on interest rate derivative instruments recognized in other comprehensive income	$ 1.6	$ 2.5
Designated as Hedging Instrument [Member] \| Interest Rate Swaps [Member]
Derivative [Line Items]
Gain (loss) on interest rate derivative instruments recognized in other comprehensive income	$ 1.6	$ 2.5

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