Income Taxes - Reconciliation of Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income taxes computed at the federal statutory rate	21.00%	21.00%	24.50%
State taxes, net of federal benefit	2.90%	2.90%	2.60%
Tax Act transitional impact	0.00%	0.00%	7.20%
Permanent items	1.00%	0.10%	(2.20%)
Net income attributable to noncontrolling interest	(10.10%)	(10.10%)	(10.80%)
Total	14.80%	13.90%	21.30%
Income tax expense related to deferred tax assets			$ 19,817
Permanent item related to tax receivable agreement			$ 24,710