Retirement Plan and Other Post-Retirement Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Thousands		Sep. 30, 2024		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash		$ (9,985)		$ 3,075
Total plan assets		(823,985)		(784,712)	$ (845,205)	$ (1,095,729)
Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		917,652		854,956
Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	37,984		37,611
Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	38,234		36,088
Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	681,755	[4]	612,820
Retirement Plan | Domestic Fixed Income | Derivative
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		8,500
Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5]	8,587		7,778
Retirement Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[6]	109,383		123,859
Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		41,709		36,800
Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		833,970		781,637
Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		83,682		73,319
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Miscellaneous Accruals, Interest Receivables, and Non-Interest Cash		1,243		10,795
Total plan assets		(496,065)		(455,702)	$ (461,438)	$ (575,565)
Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		14,274		9,637
Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		494,822		444,907
Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		83,682		73,319
Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		411,140		371,588
Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		82,814		72,285
Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		314,052		289,666
Level 1 | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		37,984		37,611
Level 1 | Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	37,984		37,611
Level 1 | Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0		0
Level 1 | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	0	[4]	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		34,484		34,399
Level 1 | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		3,500		3,212
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 Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		317,552		292,878
Level 1 | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		3,500		3,212
Level 1 | Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		314,052		289,666
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		314,052		289,666
Level 2 | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		642,187		564,282
Level 2 | Retirement Plan | Domestic Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[1]	0		0
Level 2 | Retirement Plan | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0		0
Level 2 | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	633,600	[4]	556,504
Level 2 | Retirement Plan | International Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[5]	8,587		7,778
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		583,021		516,098
Level 2 | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		59,166		48,184
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 Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		59,166		48,184
Level 2 | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals		59,166		48,184
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 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 | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2]	0		0
Level 3 | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3]	0	[4]	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 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
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
Measured at NAV | Retirement Plan | Total Retirement Plan Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	237,481		253,063
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 | Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[2],[7]	38,234		36,088
Measured at NAV | Retirement Plan | Domestic Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[3],[7]	48,155	[4]	56,316
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]	109,383		123,859
Measured at NAV | Retirement Plan | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	41,709		36,800
Measured at NAV | Retirement Plan | Total Retirement Plan Investments (excluding 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	216,465		231,140
Measured at NAV | Retirement Plan | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	21,016		21,923
Measured at NAV | Other Post-Retirement Benefits | Cash Held in Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	14,274		9,637
Measured at NAV | Other Post-Retirement Benefits | Total Investments (including 401(h) Investments)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	118,104		103,845
Measured at NAV | Other Post-Retirement Benefits | 401(h) Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	21,016		21,923
Measured at NAV | Other Post-Retirement Benefits | Total VEBA Trust Investments
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	97,088		81,922
Measured at NAV | Other Post-Retirement Benefits | Collective Trust Funds — Global Equities
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	82,814		72,285
Measured at NAV | Other Post-Retirement Benefits | Exchange Traded Funds — Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Plan assets, subtotals	[7]	$ 0		$ 0

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

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

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

[4] Domestic Fixed Income securities include $8.5 million of derivative instruments used as part of the Company’s overall liability-driven investment strategy as a way to assist in matching the duration of the assets of the Retirement Plan investments with its liability to plan participants.

[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 partnership.

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