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Pension and Retirement Plans - Components of net periodic benefit costs (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2024
Mar. 31, 2023
Pension:        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost $ 112 $ 108 $ 227 $ 218
Expected return on plan assets (146) (140) (296) (282)
Amortization of past service costs 2 2 4 4
Amortization of net (gain) loss (2) (1) (3) (2)
Net periodic (benefit) cost (34) (31) (68) (62)
Pension: | U.K..        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 109 105 221 211
Expected return on plan assets (146) (140) (296) (282)
Amortization of past service costs 2 2 4 4
Amortization of net (gain) loss 0 0 0 0
Net periodic (benefit) cost (35) (33) (71) (67)
Pension: | U.S.        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 3 3 6 7
Expected return on plan assets 0 0 0 0
Amortization of past service costs 0 0 0 0
Amortization of net (gain) loss (2) (1) (3) (2)
Net periodic (benefit) cost 1 2 3 5
Post Retirement:        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 6 6 12 12
Interest cost 16 16 32 31
Amortization of net (gain) loss (43) (49) (86) (98)
Net periodic (benefit) cost (21) (27) (42) (55)
Post Retirement: | U.K..        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0 0 0 0
Interest cost 0 0 0 0
Amortization of net (gain) loss 0 0 0 0
Net periodic (benefit) cost 0 0 0 0
Post Retirement: | U.S.        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 6 6 12 12
Interest cost 16 16 32 31
Amortization of net (gain) loss (43) (49) (86) (98)
Net periodic (benefit) cost $ (21) $ (27) $ (42) $ (55)