Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Accounting Policies [Abstract]
Estimated useful life of definite-lived intangible assets, maximum	15 years
Accumulated impairment losses	$ 0	$ 0
Provision for goodwill or other intangible asset impairments	$ 0	$ 0	$ 0