Income Taxes - Reconciliation of Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income taxes computed at the federal statutory rate	21.00%	24.50%	35.00%
State taxes, net of federal benefit	2.90%	2.60%	2.50%
Tax Cuts and Jobs Act transitional impact	0.00%	7.20%	0.00%
Permanent items	0.10%	(2.20%)	0.00%
Net income attributable to noncontrolling interest	(10.10%)	(10.80%)	(16.90%)
Total	13.90%	21.30%	20.60%
Income tax expense related to deferred tax assets		$ 19,817
Permanent item related to tax receivable agreement		$ 24,710