UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 180,004,000	$ 261,185,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	198,536,000	192,910,000
Asset impairment charges	0	12,097,000
Provision for credit loss	90,000	3,222,000
Stock-based compensation	16,101,000	15,704,000
(Gain) loss on investment securities	287,000	(24,661,000)
Gain on reimbursement of drilling equipment	(14,955,000)	(27,298,000)
Other gain on sale of assets	(12,000)	(4,898,000)
Deferred income tax expense (benefit)	(15,933,000)	3,165,000
Other	1,630,000	1,780,000
Change in assets and liabilities
Accounts receivable	(23,060,000)	(70,680,000)
Inventories of materials and supplies	(13,796,000)	(12,116,000)
Prepaid expenses and other	2,862,000	(16,387,000)
Other noncurrent assets	(9,369,000)	(1,060,000)
Accounts payable	17,505,000	33,610,000
Accrued liabilities	(15,851,000)	(42,614,000)
Deferred income tax liability	133,000	(711,000)
Other noncurrent liabilities	(5,655,000)	3,006,000
Net cash provided by operating activities	318,517,000	326,254,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(254,711,000)	(181,479,000)
Purchase of short-term investments	(74,749,000)	(64,418,000)
Purchase of long-term investments	(8,013,000)	(18,771,000)
Proceeds from sale of short-term investments	87,122,000	97,744,000
Insurance proceeds from involuntary conversion	4,980,000	0
Proceeds from asset sales	20,898,000	47,718,000
Net cash used in investing activities	(224,473,000)	(119,206,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Dividends paid	(84,371,000)	(102,941,000)
Payments for employee taxes on net settlement of equity awards	(12,176,000)	(14,410,000)
Share repurchases	(51,302,000)	(145,013,000)
Other	(250,000)	(790,000)
Net cash used in financing activities	(148,099,000)	(263,154,000)
Net decrease in cash and cash equivalents and restricted cash	(54,055,000)	(56,106,000)
Cash and cash equivalents and restricted cash, beginning of period	316,238,000	269,009,000
Cash and cash equivalents and restricted cash, end of period	262,183,000	212,903,000
Cash paid/(received) during the period:
Interest paid	8,047,000	8,919,000
Income tax paid	81,294,000	118,090,000
Income tax received	0	(25,687,000)
Payments for operating leases	6,714,000	5,970,000
Non-cash operating and investing activities:
Change in accounts payable and accrued liabilities related to purchases of property, plant and equipment	$ (15,716,000)	$ 601,000