Summary of Significant Accounting Policies - Concentration of Credit Risk (Details)	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Top Ten Largest Customers \| Credit Concentration Risk \| Sales Revenue, Net
Concentration Risk [Line Items]
Concentration risk, percentage (as a percent)	28.00%	34.00%	35.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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