Goodwill - Summary of Changes to the Carrying Amount of Goodwill (Details) - USD ($)	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Goodwill [Roll Forward]
Balance, beginning of period	$ 739,023,000	$ 739,023,000
Goodwill acquired during the period	1,539,000	0
Goodwill impairment during the period	(740,562,000)	0	$ 0
Balance, end of period	$ 0	$ 739,023,000	$ 739,023,000

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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