Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income tax computed at statutory rate	$ 14,971	$ (24,494)	$ 12,112
State income taxes, net of federal benefit	1,300	(590)	111
Deferred rate change	260	(88)	110,071
Decrease in valuation allowance - other	0	0	(27,370)
Changes in uncertain tax positions	(2)	(7)	598
Permanent differences	2,177	2,908	2,133
Tax credits	(939)	(14,902)	(3,174)
Other, net	206	(44)	3
Total expense (benefit)	$ 17,973	$ (37,217)	94,484
Income Tax Contingency [Line Items]
Federal tax rate for fiscal year provision, percent	24.50%
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Valuation allowances and reserves, adjustments			$ 27,400