Pension and Retirement Plans - Accrued benefit liability (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Liability, Defined Benefit Plan [Abstract]
Current accrued benefit liability	$ 321	$ 335
Non-current accrued benefit liability	6,471	6,904
Total accrued benefit liability	6,792	7,239
Accumulated other comprehensive income	6,120	$ 6,302
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	152
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 400