Retirement Plan and Other Post-Retirement Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Thousands		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Total plan assets		$ 784,712	$ 845,205	$ 1,095,729	$ 1,016,796
Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		854,956	912,172
Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	37,611	41,633
Retirement Plan | International Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0	1,363
Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	36,088	44,434
Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[4]	612,820	658,833
Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5]	7,778	7,782
Retirement Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[6]	123,859	140,739
Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		36,800	17,388
Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		781,637	839,128
Retirement Plan | Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		3,075	6,077
Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		73,319	73,044
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total plan assets		455,702	461,438	$ 575,565	$ 547,885
Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		9,637	10,635
Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		371,588	385,770
Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		72,285	104,554
Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		289,666	270,581
Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		444,907	458,814
Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		73,319	73,044
Other Post-Retirement Benefits | Miscellaneous Accruals (including Current and Deferred Taxes, Claims Incurred But Not Reported, Administrative)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		10,795	2,624
Level 1 | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		37,611	41,633
Level 1 | Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	37,611	41,633
Level 1 | Retirement Plan | International Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0	0
Level 1 | Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	0	0
Level 1 | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[4]	0	0
Level 1 | Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5]	0	0
Level 1 | Retirement Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[6]	0	0
Level 1 | Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 1 | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		34,399	38,323
Level 1 | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		3,212	3,310
Level 1 | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 1 | Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		289,666	270,581
Level 1 | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 1 | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		289,666	270,581
Level 1 | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		292,878	273,891
Level 1 | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		3,212	3,310
Level 2 | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		564,282	587,388
Level 2 | Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	0	0
Level 2 | Retirement Plan | International Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0	0
Level 2 | Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	0	0
Level 2 | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[4]	556,504	579,606
Level 2 | Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5]	7,778	7,782
Level 2 | Retirement Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[6]	0	0
Level 2 | Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 2 | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		516,098	540,694
Level 2 | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		48,184	46,694
Level 2 | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 2 | Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 2 | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 2 | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 2 | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		48,184	46,694
Level 2 | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		48,184	46,694
Level 3 | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	0	0
Level 3 | Retirement Plan | International Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0	0
Level 3 | Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	0	0
Level 3 | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[4]	0	0
Level 3 | Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5]	0	0
Level 3 | Retirement Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[6]	0	0
Level 3 | Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Level 3 | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		0	0
Measured at NAV | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	253,063	283,151
Measured at NAV | Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1],[7]	0	0
Measured at NAV | Retirement Plan | International Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2],[7]	0	1,363
Measured at NAV | Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3],[7]	36,088	44,434
Measured at NAV | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[4],[7]	56,316	79,227
Measured at NAV | Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5],[7]	0	0
Measured at NAV | Retirement Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[6],[7]	123,859	140,739
Measured at NAV | Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	36,800	17,388
Measured at NAV | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	231,140	260,111
Measured at NAV | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	21,923	23,040
Measured at NAV | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[8]	9,637	10,635
Measured at NAV | Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[8]	81,922	115,189
Measured at NAV | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[8]	72,285	104,554
Measured at NAV | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[8]	0	0
Measured at NAV | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[8]	103,845	138,229
Measured at NAV | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[8]	$ 21,923	$ 23,040

[1] Domestic Equities include mostly collective trust funds, common stock, and exchange traded funds.

[2] International Equities are comprised of collective trust funds.

[3] Global Equities are comprised of collective trust funds.

[4] Domestic Fixed Income securities include mostly collective trust funds, corporate/government bonds and mortgages, and exchange traded funds.

[5] International Fixed Income securities are comprised mostly of corporate/government bonds.

[6] Real Estate consists of investments held in a collective trust fund and a real estate investment trust.

[7] Reflects the authoritative guidance related to investments measured at net asset value (NAV).

[8] Reflects the authoritative guidance related to investments measured at net asset value (NAV).