Pension and Retirement Plans - Components of net periodic benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in minimum liability included in other comprehensive income (loss)	$ (150)	$ 1,063
U.K.
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in minimum liability included in other comprehensive income (loss)	(231)	914
U.S.
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in minimum liability included in other comprehensive income (loss)	81	149
Pension
Defined Benefit Plan Disclosure [Line Items]
Interest cost	278	386
Expected return on plan assets	(289)	(327)
Amortization of past service costs	7
Amortization of net gain (loss)	193	146
Net periodic benefit cost	189	205
Increase (decrease) in minimum liability included in other comprehensive income (loss)	(229)	932
Pension | International
Defined Benefit Plan Disclosure [Line Items]
Interest cost	263	362
Pension | U.K.
Defined Benefit Plan Disclosure [Line Items]
Interest cost	263	362
Expected return on plan assets	(289)	(327)
Amortization of past service costs	7
Amortization of net gain (loss)	190	149
Net periodic benefit cost	171	184
Increase (decrease) in minimum liability included in other comprehensive income (loss)	(231)	914
Pension | U.S.
Defined Benefit Plan Disclosure [Line Items]
Interest cost	15	24
Amortization of net gain (loss)	3	(3)
Net periodic benefit cost	18	21
Increase (decrease) in minimum liability included in other comprehensive income (loss)	2	18
Post Retirement
Defined Benefit Plan Disclosure [Line Items]
Service cost	38	34
Interest cost	46	53
Amortization of net gain (loss)	24	(20)
Net periodic benefit cost	108	67
Increase (decrease) in minimum liability included in other comprehensive income (loss)	79	131
Post Retirement | U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	38	34
Interest cost	46	53
Amortization of net gain (loss)	24	(20)
Net periodic benefit cost	108	67
Increase (decrease) in minimum liability included in other comprehensive income (loss)	$ 79	$ 131