Pension Plan and Retirement Benefits - Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Pension Plans
Asset allocation
Total pension plan assets	$ 225,917	$ 222,273	$ 224,094
Defined Benefit Pension Plans | NAV
Asset allocation
Total pension plan assets	225,917	222,273
Pension Plans Defined Benefit Us [Member]
Asset allocation
Total pension plan assets	204,678	201,896
Pension Plans Defined Benefit Us [Member] | U.S. common stock mutual funds
Asset allocation
Total pension plan assets	67,954	72,947
Pension Plans Defined Benefit Us [Member] | Bonds
Asset allocation
Total pension plan assets	81,871	80,250
Pension Plans Defined Benefit Us [Member] | U.S. common stock
Asset allocation
Total pension plan assets	30,292	32,547
Pension Plans Defined Benefit Us [Member] | International equity
Asset allocation
Total pension plan assets	24,561	16,152
Pension Plans Defined Benefit Us [Member] | NAV
Asset allocation
Total pension plan assets	204,678	201,896
Pension Plans Defined Benefit Us [Member] | NAV | U.S. common stock mutual funds
Asset allocation
Total pension plan assets	67,954	72,947
Pension Plans Defined Benefit Us [Member] | NAV | Bonds
Asset allocation
Total pension plan assets	81,871	80,250
Pension Plans Defined Benefit Us [Member] | NAV | U.S. common stock
Asset allocation
Total pension plan assets	30,292	32,547
Pension Plans Defined Benefit Us [Member] | NAV | International equity
Asset allocation
Total pension plan assets	24,561	16,152
Pension Plans Defined Benefit Non Us [Member]
Asset allocation
Total pension plan assets	21,239	20,377
Pension Plans Defined Benefit Non Us [Member] | Bonds
Asset allocation
Total pension plan assets	12,106	9,781
Pension Plans Defined Benefit Non Us [Member] | Equity
Asset allocation
Total pension plan assets	6,585	8,150
Pension Plans Defined Benefit Non Us [Member] | Other
Asset allocation
Total pension plan assets	2,548	2,446
Pension Plans Defined Benefit Non Us [Member] | NAV
Asset allocation
Total pension plan assets	21,239	20,377
Pension Plans Defined Benefit Non Us [Member] | NAV | Bonds
Asset allocation
Total pension plan assets	12,106	9,781
Pension Plans Defined Benefit Non Us [Member] | NAV | Equity
Asset allocation
Total pension plan assets	6,585	8,150
Pension Plans Defined Benefit Non Us [Member] | NAV | Other
Asset allocation
Total pension plan assets	$ 2,548	$ 2,446