CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net income	$ 71,448	$ 40,580
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	64,192	43,889
Accretion on Asset Retirement Obligations - Liabilities	255	0
Provision for doubtful accounts	185	(314)
Share-based compensation expense	8,997	11,708
Deferred income tax expense (benefit)	(7,117)	(13,811)
Non-cash foreign exchange (gain) loss	325	354
Loss (Gain) on disposal of property, plant and equipment	(69)	(58)
Non-cash charge on inventory step up of acquired inventory sold	0	14,827
Amortization of debt issuance costs	1,565	1,286
Other	959	2,337
Changes in operating assets and liabilities:
Accounts receivable	(6,386)	9,445
Inventories	(13,103)	(21,100)
Prepaid expenses and other assets	5,020	5,415
Accounts payable	(1,102)	(429)
Accrued expenses, income taxes payable and other liabilities	(12,830)	(40,601)
Net cash provided by operating activities	112,339	53,528
Net Cash Provided by (Used in) Investing Activities [Abstract]
Additions to property, plant and equipment	(59,192)	(19,472)
Proceeds from sales of property, plant and equipment	1,587	1,210
Acquisition of a business, net of cash acquired	0	(1,182,186)
Cash settlement of life insurance policy	0	3,959
Net cash used in investing activities	(57,605)	(1,196,489)
Cash flows from financing activities:
Repayment of long-term debt	(17,988)	(45,000)
Repurchases of common stock	(19,513)	(4,695)
Proceeds from issuance of long-term debt	0	1,062,337
Proceeds from revolving line of credit	150,000	0
Debt issuance costs	0	(18,745)
Principal payments under financing lease obligations	(4)
Proceeds from issuance of stock	10,334	10,361
Dividends paid	(24,752)	(21,934)
Net cash provided by financing activities	98,077	982,324
Effect of exchange rate changes on cash	(604)	(24)
Increase (decrease) in cash and cash equivalents	152,207	(160,661)
Cash and cash equivalents at beginning of period	188,495	352,921
Cash and cash equivalents at end of period	340,702	192,260
Supplemental Cash Flow Information:
Purchases of property, plant and equipment in accrued liabilities and accounts payable at the end of the period	13,841	2,425
Equity consideration related to the acquisition of KMG Chemicals, Inc	0	$ 331,048
Cash paid during the period for lease liabilities	3,825
Right of use asset obtained in exchange for lease liabilities	$ 3,334