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Summary of Significant Accounting Policies - Impairment of Long-Lived Assets and Cost Basis Investments (Details) - USD ($)
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Accounting Policies [Abstract]      
Provisions for impairments $ 0 $ 0 $ 0
Other-than-temporary impairment of an investment in a private equity partnership pre-tax loss $ 0 $ 11,000,000 $ 0