CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net income	$ 142,828	$ 39,215	$ 110,043
Adjustments to reconcile Net income to net cash provided by operating activities:
Depreciation and amortization	127,737	98,592	25,876
Accretion on Asset Retirement Obligations	599	530	0
Provision for doubtful accounts	443	432	185
Share-based compensation expense	16,396	18,227	18,517
Deemed repatriation transition tax	0	0	11,340
Deferred income tax expense (benefit)	(11,267)	(27,150)	10,835
Non-cash foreign exchange (gain) loss	(266)	839	(873)
(Gain) on sale of assets	(71)	(36)	(865)
Impairment of assets	2,314	67,372	0
Realized loss on the sale of available-for-sale securities	0	0	96
Non-cash charge on inventory step up of acquired inventory sold	0	14,869	0
Amortization of debt issuance costs	3,123	2,884	0
Other	(1,239)	(1,362)	1,666
Changes in operating assets and liabilities:
Accounts receivable	13,075	(6,156)	(12,068)
Inventories	(12,337)	(20,993)	(442)
Prepaid expenses and other assets	8,645	6,830	(5,818)
Accounts payable	(2,861)	1,163	128
Accrued expenses, income taxes payable and other liabilities	165	(20,275)	10,245
Net cash provided by operating activities	287,284	174,981	168,865
Cash flows from investing activities:
Additions to property, plant and equipment	(125,839)	(55,972)	(21,308)
Proceeds from the sale of assets	1,587	1,224	3,027
Acquisition of a business, net of cash acquired	0	(1,182,187)	0
Cash settlement of life insurance policy	0	3,959	0
Purchases of available-for-sale securities	0	0	(209,048)
Proceeds from the sale and maturities of available-for-sale securities	0	0	214,460
Settlement of net investment hedge	0	0	(9,882)
Net cash used in investing activities	(124,252)	(1,232,976)	(22,751)
Cash flows from financing activities:
Repayment of long-term debt	(23,313)	(105,326)	(144,375)
Dividends paid	(50,383)	(46,324)	(30,730)
Repurchases of common stock	(38,238)	(14,720)	(44,288)
Proceeds from issuance of stock	14,427	17,210	23,031
Proceeds from issuance of long-term debt	0	1,062,337	0
Proceeds from revolving line of credit	150,000	0	0
Repayment on revolving line of credit	(150,000)	0	0
Debt issuance costs	0	(18,745)	0
Other financing activities	(149)	0	(1,200)
Net cash provided by (used in) financing activities	(97,656)	894,432	(197,562)
Effect of exchange rate changes on cash	3,483	(863)	6,479
Increase (decrease) in cash and cash equivalents	68,859	(164,426)	(44,969)
Cash and cash equivalents at beginning of year	188,495	352,921	397,890
Cash and cash equivalents at end of year	257,354	188,495	352,921
Supplemental disclosure of cash flow information:
Cash paid for income taxes (net of refunds received)	44,535	35,432	20,345
Cash paid for interest	45,281	39,181	2,464
Purchases of property, plant and equipment in accrued liabilities and accounts payable at the end of the period	5,365	8,690	1,975
Equity consideration related to the acquisition of KMG Chemicals, Inc	0	331,048	0
Cash paid during the period for lease liabilities	7,554	0	0
Right of use asset obtained in exchange for lease liabilities	$ 7,435	$ 0	$ 0