Summary of Significant Accounting Policies - Receivable Allowances (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
SEC Schedule, 12-09, Allowance, Credit Loss [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Valuation Allowances and Reserves, Beginning Balance	$ 9,823	$ 11,938	$ 8,349
Valuation Allowances and Reserves, Bad debt provision	2,375	2,377	3,333
Valuation Allowances and Reserves, Write-offs, net of recoveries	(1,536)	(4,492)	256
Valuation Allowances and Reserves, Adjustments	0	0	0
Valuation Allowances and Reserves, Ending Balance	10,662	9,823	11,938
Allowance for Sales Returns [Member] (Deprecated 2018-01-31)
Allowance for Doubtful Accounts Receivable [Roll Forward]
Valuation Allowances and Reserves, Beginning Balance	6,145	29,541	3,166
Valuation Allowances and Reserves, Bad debt provision	0	0	0
Valuation Allowances and Reserves, Write-offs, net of recoveries	0	0	0
Valuation Allowances and Reserves, Adjustments	(765)	(23,396)	26,375
Valuation Allowances and Reserves, Ending Balance	$ 5,380	$ 6,145	$ 29,541