Income Taxes - Deferred tax assets (Details) - USD ($)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Deferred tax assets attributable to:
Foreign loss carryforwards	$ 35,091,000	$ 27,097,000
Accrued liabilities	8,871,000	12,568,000
Share-based compensation expense	8,988,000	9,494,000
U.S. foreign tax credits	11,199,000	8,807,000
U.S. federal social security tax deferral	4,038,000
Inventory adjustments	2,131,000	1,242,000
Other	1,101,000	651,000
Total deferred tax assets	71,419,000	59,859,000
Valuation allowance	(50,543,000)	(38,287,000)
Total deferred tax assets, net	20,876,000	21,572,000
Deferred tax liabilities attributable to:
Depreciation and amortization	107,672,000	94,920,000
Net deferred tax liability	86,796,000	73,348,000
Domestic earnings before provision for income taxes	168,000,000.0	328,300,000	$ 300,400,000
Foreign earnings before provision for income (loss) taxes	(8,000,000.0)	33,900,000	$ 28,000,000.0
Deferred taxes on accumulated undistributed earnings of our foreign operations	0
Total operating loss carry-forward	128,200,000	97,300,000
Operating loss carry-forward, subject to valuation allowance	111,700,000	79,000,000.0
Amount of operating loss carry-forwards with an expiration date	6,200,000
Amount of operating loss carry-forwards without an expiration date	122,000,000.0
Total tax credit carryforwards	13,800,000	11,200,000
Expiring tax credit carryforwards	1,200,000
Non-expiring tax credit carryforwards	1,400,000
Total Tax credit carryforwards, subject to a valuation allowance	12,600,000	$ 10,100,000
U.S. foreign
Deferred tax liabilities attributable to:
Expiring tax credit carryforwards	$ 11,200,000