Fair Value Measurements - Recurring Fair Value Measures of Assets and Liabilities (Details) - USD ($) $ in Thousands		Sep. 30, 2023	Sep. 30, 2022		Sep. 30, 2021	Sep. 30, 2020
Assets:
Cash Equivalents — Money Market Mutual Funds	[1]	$ 39,332	$ 35,015
Hedging Collateral Deposits		$ 0	$ 91,670	[1]	$ 88,610	$ 0
Derivative Financial Instruments:
Derivative Asset, Statement of Financial Position [Extensible Enumeration]		Fair Value of Derivative Financial Instruments	Fair Value of Derivative Financial Instruments
Total Assets	[1]	$ 121,553	$ 188,714
Derivative Financial Instruments:
Derivative Liability, Statement of Financial Position [Extensible Enumeration]		Fair Value of Derivative Financial Instruments	Fair Value of Derivative Financial Instruments
Total Liabilities	[1]	$ 31,009	$ 785,659
Total Net Assets/(Liabilities)	[1]	90,544	(596,945)
Over the Counter Swaps — Gas
Derivative Financial Instruments:
Derivative Asset	[1]	28,292	999
Derivative Financial Instruments:
Derivative Liability	[1]	30,803	513,286
Over the Counter No Cost Collars — Gas
Derivative Financial Instruments:
Derivative Asset	[1]	16,221
Derivative Financial Instruments:
Derivative Liability	[1]	205	270,453
Contingent Consideration for Asset Sale
Derivative Financial Instruments:
Derivative Asset	[1]	7,277	8,176
Foreign Currency Contracts
Derivative Financial Instruments:
Derivative Asset	[1]	(1,303)	0
Derivative Financial Instruments:
Derivative Liability	[1]	1	1,920
Balanced Equity Mutual Fund
Derivative Financial Instruments:
Other Investments	[1]	15,837	19,506
Fixed Income Mutual Fund
Derivative Financial Instruments:
Other Investments	[1]	15,897	33,348
Level 1
Assets:
Cash Equivalents — Money Market Mutual Funds		39,332	35,015
Hedging Collateral Deposits			91,670
Derivative Financial Instruments:
Total Assets		71,066	179,539
Derivative Financial Instruments:
Total Liabilities		0	0
Total Net Assets/(Liabilities)		71,066	179,539
Level 1 | Over the Counter Swaps — Gas
Derivative Financial Instruments:
Derivative Asset		0	0
Derivative Financial Instruments:
Derivative Liability		0	0
Level 1 | Over the Counter No Cost Collars — Gas
Derivative Financial Instruments:
Derivative Asset		0
Derivative Financial Instruments:
Derivative Liability		0	0
Level 1 | Contingent Consideration for Asset Sale
Derivative Financial Instruments:
Derivative Asset		0	0
Level 1 | Foreign Currency Contracts
Derivative Financial Instruments:
Derivative Asset		0	0
Derivative Financial Instruments:
Derivative Liability		0	0
Level 1 | Balanced Equity Mutual Fund
Derivative Financial Instruments:
Other Investments		15,837	19,506
Level 1 | Fixed Income Mutual Fund
Derivative Financial Instruments:
Other Investments		15,897	33,348
Level 2
Assets:
Cash Equivalents — Money Market Mutual Funds		0	0
Hedging Collateral Deposits			0
Derivative Financial Instruments:
Total Assets		104,193	13,481
Derivative Financial Instruments:
Total Liabilities		84,715	789,965
Total Net Assets/(Liabilities)		19,478	(776,484)
Level 2 | Over the Counter Swaps — Gas
Derivative Financial Instruments:
Derivative Asset		65,800	5,177
Derivative Financial Instruments:
Derivative Liability		68,311	517,464
Level 2 | Over the Counter No Cost Collars — Gas
Derivative Financial Instruments:
Derivative Asset		30,966
Derivative Financial Instruments:
Derivative Liability		14,950	270,453
Level 2 | Contingent Consideration for Asset Sale
Derivative Financial Instruments:
Derivative Asset		7,277	8,176
Level 2 | Foreign Currency Contracts
Derivative Financial Instruments:
Derivative Asset		150	128
Derivative Financial Instruments:
Derivative Liability		1,454	2,048
Level 2 | Balanced Equity Mutual Fund
Derivative Financial Instruments:
Other Investments		0	0
Level 2 | Fixed Income Mutual Fund
Derivative Financial Instruments:
Other Investments		0	0
Level 3
Assets:
Cash Equivalents — Money Market Mutual Funds		0	0
Hedging Collateral Deposits			0
Derivative Financial Instruments:
Total Assets		0	0
Derivative Financial Instruments:
Total Liabilities		0	0
Total Net Assets/(Liabilities)		0	0
Level 3 | Over the Counter Swaps — Gas
Derivative Financial Instruments:
Derivative Asset		0	0
Derivative Financial Instruments:
Derivative Liability		0	0
Level 3 | Over the Counter No Cost Collars — Gas
Derivative Financial Instruments:
Derivative Asset		0
Derivative Financial Instruments:
Derivative Liability		0	0
Level 3 | Contingent Consideration for Asset Sale
Derivative Financial Instruments:
Derivative Asset		0	0
Level 3 | Foreign Currency Contracts
Derivative Financial Instruments:
Derivative Asset		0	0
Derivative Financial Instruments:
Derivative Liability		0	0
Level 3 | Balanced Equity Mutual Fund
Derivative Financial Instruments:
Other Investments		0	0
Level 3 | Fixed Income Mutual Fund
Derivative Financial Instruments:
Other Investments		0	0
Netting Adjustments
Assets:
Cash Equivalents — Money Market Mutual Funds	[1]	0	0
Hedging Collateral Deposits	[1]		0
Derivative Financial Instruments:
Total Assets	[1]	(53,706)	(4,306)
Derivative Financial Instruments:
Total Liabilities	[1]	(53,706)	(4,306)
Total Net Assets/(Liabilities)	[1]	0	0
Netting Adjustments | Over the Counter Swaps — Gas
Derivative Financial Instruments:
Derivative Asset	[1]	(37,508)	(4,178)
Derivative Financial Instruments:
Derivative Liability	[1]	(37,508)	(4,178)
Netting Adjustments | Over the Counter No Cost Collars — Gas
Derivative Financial Instruments:
Derivative Asset	[1]	(14,745)
Derivative Financial Instruments:
Derivative Liability	[1]	(14,745)	0
Netting Adjustments | Contingent Consideration for Asset Sale
Derivative Financial Instruments:
Derivative Asset	[1]	0	0
Netting Adjustments | Foreign Currency Contracts
Derivative Financial Instruments:
Derivative Asset	[1]	(1,453)	(128)
Derivative Financial Instruments:
Derivative Liability	[1]	(1,453)	(128)
Netting Adjustments | Balanced Equity Mutual Fund
Derivative Financial Instruments:
Other Investments	[1]	0	0
Netting Adjustments | Fixed Income Mutual Fund
Derivative Financial Instruments:
Other Investments	[1]	$ 0	$ 0

[1] Netting Adjustments represent the impact of legally-enforceable master netting arrangements that allow the Company to net gain and loss positions held with the same counterparties. The net asset or net liability for each counterparty is recorded as an asset or liability on the Company’s balance sheet.