Income Taxes - Reconciliation from US Federal Statutory Tax Rate to Effective Tax Rate (Details)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
U.S. federal statutory tax rate	24.50%	35.00%	35.00%
Difference in tax rate due to:
State income taxes, net of federal benefit	4.70%	5.00%	5.20%
Effect of tax rate changes - TCJA	(3.80%)	0.00%	0.00%
Excess tax benefits on share-based payments	(0.50%)	(0.90%)	0.00%
Other, net	(1.40%)	(0.80%)	0.20%
Effective tax rate	23.50%	38.30%	40.40%