Supplementary Information for Oil and Gas Producing Activities - Results of Operations for Producing Activities (Details) - USD ($)		12 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Reserve Quantities [Line Items]
Operating Revenues	[1]	$ 1,038,760,000	$ 1,881,680,000	$ 915,668,000
Production/Lifting Costs		253,555,000	283,914,000	267,316,000
Franchise/Ad Valorem Taxes		17,532,000	25,112,000	22,128,000
Purchased Emission Allowance Expense		0	1,305,000	2,940,000
Accretion Expense		5,673,000	7,530,000	7,743,000
Depreciation, Depletion and Amortization ($0.63, $0.57 and $0.54 per Mcfe of production, respectively)		235,694,000	202,418,000	177,055,000
Impairment of Oil and Gas Producing Properties		0	0	76,152,000
Income Tax Expense		145,574,000	368,925,000	98,593,000
Results of Operations for Producing Activities (excluding corporate overheads and interest charges)		380,732,000	992,476,000	263,741,000
Revenues from sales to affiliates		0	0	0
Transfers to operations		1,957,000	5,696,000	3,061,000
Depreciation, Depletion and Amortization (per Mcfe of production)		0.63	0.57	0.54
Gas
Reserve Quantities [Line Items]
Gas (includes transfers to operations of $1,957, $5,696 and $3,061, respectively)	[2]	1,036,499,000	1,730,723,000	780,477,000
Oil, Condensate and Other Liquids
Reserve Quantities [Line Items]
Operating Revenues		$ 2,261,000	$ 150,957,000	$ 135,191,000

[1] Exclusive of hedging gains and losses. See further discussion in Note J — Financial Instruments.

[2] There were no revenues from sales to affiliates for all years presented.