Income Taxes - Deferred tax assets (Details) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets attributable to:
Foreign loss carryforwards	$ 27,097,000	$ 28,612,000
Accrued liabilities	12,248,000	15,676,000
Share-based compensation expense	9,494,000	10,762,000
U.S. foreign tax credits	8,807,000	8,807,000
Unrecognized tax benefits	320,000	322,000
Inventory adjustments	1,242,000
Other	651,000	442,000
Total deferred tax assets	59,859,000	64,621,000
Valuation allowance	(38,287,000)	(40,906,000)
Total deferred tax assets, net	21,572,000	23,715,000
Deferred tax liabilities attributable to:
Depreciation and amortization	94,920,000	92,531,000
Inventory adjustments		673,000
Total deferred tax liabilities	94,920,000	93,204,000
Net deferred tax liability	73,348,000	69,489,000
Domestic earnings before provision for income taxes	328,300,000	300,400,000	$ 332,500,000
Foreign earnings before provision for income taxes	33,900,000	28,000,000.0	$ 13,200,000
Deferred taxes on accumulated undistributed earnings of our foreign operations	0
Total operating loss carry-forward	97,300,000	103,000,000.0
Operating loss carry-forward, subject to valuation allowance	79,000,000.0	88,600,000
Amount of operating loss carry-forwards with an expiration date	7,700,000
Amount of operating loss carry-forwards without an expiration date	89,500,000
Total tax credit carryforwards	11,200,000	11,600,000
Expiring tax credit carryforwards	1,100,000
Non-expiring tax credit carryforwards	1,300,000
Total Tax credit carryforwards, subject to a valuation allowance	10,100,000	$ 10,200,000
U.S. foreign
Deferred tax liabilities attributable to:
Expiring tax credit carryforwards	$ 8,800,000