Intangible Assets - Schedule of Intangible Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Intangible Assets Net Excluding Goodwill [Abstract]
Intangible assets subject to amortization, beginning balance	$ 15,363
Intangible assets subject to amortization, Amortization	(1,700)	$ (1,700)	$ (1,700)
Intangible assets subject to amortization, ending balance	$ 13,663	$ 15,363

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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