Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance, beginning of year	$ 177,663	$ 141,950	$ 279,683
Net release of valuation allowance	0	(1,772)	(2,791)
Net increase (decrease) in deferred tax assets with a full valuation allowance	27,760	37,485	(134,942)
Valuation allowance, end of year	$ 205,423	$ 177,663	$ 141,950