Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Net changes in the liability for unrecognized tax benefits
Balance at beginning of year	$ 18,575	$ 9,942
Reductions for tax positions taken in prior years	(8,794)
Additions for tax positions taken in prior years		8,458
Recognition of benefits from expiration of statutes	(904)	(776)
Additions for tax positions related to the current year	890	951
Balance at end of year	9,767	18,575	$ 9,942
Unrecognized tax benefits from permanent tax adjustments that, if recognized, would affect the effective rate	500	700
Cash amounts paid for income taxes, net of refunds received	$ 16,600	$ 28,700	$ 15,700