Pension Plan and Retirement Benefits - Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Pension Plans
Asset allocation
Total pension plan assets	$ 222,273	$ 224,094	$ 207,963
Defined Benefit Pension Plans | NAV
Asset allocation
Total pension plan assets	222,273	224,094
Pension Plans Defined Benefit Us [Member]
Asset allocation
Total pension plan assets	201,896	204,257
Pension Plans Defined Benefit Us [Member] | U.S. common stock mutual funds
Asset allocation
Total pension plan assets	72,947	73,430
Pension Plans Defined Benefit Us [Member] | Bonds
Asset allocation
Total pension plan assets	80,250	81,702
Pension Plans Defined Benefit Us [Member] | U.S. common stock
Asset allocation
Total pension plan assets	32,547	32,784
Pension Plans Defined Benefit Us [Member] | International equity
Asset allocation
Total pension plan assets	16,152	16,341
Pension Plans Defined Benefit Us [Member] | NAV
Asset allocation
Total pension plan assets	201,896	204,257
Pension Plans Defined Benefit Us [Member] | NAV | U.S. common stock mutual funds
Asset allocation
Total pension plan assets	72,947	73,430
Pension Plans Defined Benefit Us [Member] | NAV | Bonds
Asset allocation
Total pension plan assets	80,250	81,702
Pension Plans Defined Benefit Us [Member] | NAV | U.S. common stock
Asset allocation
Total pension plan assets	32,547	32,784
Pension Plans Defined Benefit Us [Member] | NAV | International equity
Asset allocation
Total pension plan assets	16,152	16,341
Pension Plans Defined Benefit Non Us [Member]
Asset allocation
Total pension plan assets	20,377	19,837
Pension Plans Defined Benefit Non Us [Member] | Bonds
Asset allocation
Total pension plan assets	9,781	7,750
Pension Plans Defined Benefit Non Us [Member] | Equity
Asset allocation
Total pension plan assets	8,150	8,913
Pension Plans Defined Benefit Non Us [Member] | Other
Asset allocation
Total pension plan assets	2,446	3,174
Pension Plans Defined Benefit Non Us [Member] | NAV
Asset allocation
Total pension plan assets	20,377	19,837
Pension Plans Defined Benefit Non Us [Member] | NAV | Bonds
Asset allocation
Total pension plan assets	9,781	7,750
Pension Plans Defined Benefit Non Us [Member] | NAV | Equity
Asset allocation
Total pension plan assets	8,150	8,913
Pension Plans Defined Benefit Non Us [Member] | NAV | Other
Asset allocation
Total pension plan assets	$ 2,446	$ 3,174