Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income tax computed at statutory rate	$ (24,494)	$ 12,112	$ 12,052
State income taxes, net of federal benefit	(590)	111	1,287
Deferred rate change	(88)	110,071	0
Decrease in valuation allowance - other	0	(27,370)	(3,482)
Changes in uncertain tax positions	(7)	598	(685)
Stock based compensation	0	0	741
Permanent differences	2,908	2,133	496
Tax credits	(14,902)	(3,174)	(7,460)
Other, net	(44)	3	(253)
Total (benefit) / expense	$ (37,217)	$ 94,484	2,696
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	$ 3,500