Pension Plan and Retirement Benefits - Plan Assets (Details) $ in Thousands	Sep. 30, 2023 USD ($) item	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)
Defined Benefit Pension Plan
Asset allocation
Total pension plan assets	$ 219,971	$ 218,321	$ 297,703
Defined Benefit Pension Plan | Fair Value, Inputs, Level 1 [Member]
Asset allocation
Total pension plan assets	32,155	15,926
Defined Benefit Pension Plan | Fair Value, Inputs, Level 2 [Member]
Asset allocation
Total pension plan assets	154,686	158,582
Defined Benefit Pension Plan | NAV
Asset allocation
Total pension plan assets	$ 33,130	43,813
Pension Plans Defined Benefit Us [Member]
Defined Benefit Plans
Number of issuances of fixed income securities | item	301
Number of issuers of fixed income securities | item	238
Asset allocation
Total pension plan assets	$ 203,771	203,216
Pension Plans Defined Benefit Us [Member] | U.S. corporate and government bonds
Asset allocation
Total pension plan assets	181,333	174,508
Pension Plans Defined Benefit Us [Member] | Mutual Fund [Member]
Asset allocation
Total pension plan assets		16,471
Pension Plans Defined Benefit Us [Member] | U.S. common stock
Asset allocation
Total pension plan assets	17,900	6,847
Pension Plans Defined Benefit Us [Member] | International equity
Asset allocation
Total pension plan assets	4,538	5,390
Pension Plans Defined Benefit Us [Member] | Fair Value, Inputs, Level 1 [Member]
Asset allocation
Total pension plan assets	26,647	15,926
Pension Plans Defined Benefit Us [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. corporate and government bonds
Asset allocation
Total pension plan assets	26,647	15,926
Pension Plans Defined Benefit Us [Member] | Fair Value, Inputs, Level 2 [Member]
Asset allocation
Total pension plan assets	154,686	158,582
Pension Plans Defined Benefit Us [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. corporate and government bonds
Asset allocation
Total pension plan assets	154,686	158,582
Pension Plans Defined Benefit Us [Member] | NAV
Asset allocation
Total pension plan assets	22,438	28,708
Pension Plans Defined Benefit Us [Member] | NAV | Mutual Fund [Member]
Asset allocation
Total pension plan assets		16,471
Pension Plans Defined Benefit Us [Member] | NAV | U.S. common stock
Asset allocation
Total pension plan assets	17,900	6,847
Pension Plans Defined Benefit Us [Member] | NAV | International equity
Asset allocation
Total pension plan assets	4,538	5,390
Pension Plans Defined Benefit Non Us [Member]
Asset allocation
Total pension plan assets	16,200	15,105
Pension Plans Defined Benefit Non Us [Member] | Bonds [Member]
Asset allocation
Total pension plan assets	10,692	8,006
Pension Plans Defined Benefit Non Us [Member] | Money Market Funds [Member]
Asset allocation
Total pension plan assets	5,508
Pension Plans Defined Benefit Non Us [Member] | Equity
Asset allocation
Total pension plan assets		5,135
Pension Plans Defined Benefit Non Us [Member] | Other
Asset allocation
Total pension plan assets		1,964
Pension Plans Defined Benefit Non Us [Member] | Fair Value, Inputs, Level 1 [Member]
Asset allocation
Total pension plan assets	5,508
Pension Plans Defined Benefit Non Us [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Asset allocation
Total pension plan assets	5,508
Pension Plans Defined Benefit Non Us [Member] | NAV
Asset allocation
Total pension plan assets	10,692	15,105
Pension Plans Defined Benefit Non Us [Member] | NAV | Bonds [Member]
Asset allocation
Total pension plan assets	$ 10,692	8,006
Pension Plans Defined Benefit Non Us [Member] | NAV | Equity
Asset allocation
Total pension plan assets		5,135
Pension Plans Defined Benefit Non Us [Member] | NAV | Other
Asset allocation
Total pension plan assets		$ 1,964