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Defined Benefit Pension Plan - Actuarial Assumptions (Details)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Retirement Benefits [Abstract]    
Discount rate to determine net periodic (benefit) cost 1.60% 1.80%
Expected return on plan assets 2.82% 3.00%
Discount rate to determine benefit obligations 2.00% 1.60%
Rate of increases to deferred CPI linked benefits 3.10% 2.50%
Rate of increases to deferred RPI linked benefits 3.60% 3.00%