Captive Insurance Collateral - Schedule of Maturities of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Investments Debt And Equity Securities [Abstract]
Due within one year	$ 4,982
Due after one year through five years	63,714
Due after five years through ten years	1,099
Total	$ 69,795	$ 69,787

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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