Retirement Benefit Plans - Changes in the Fair Value of Level 3 Defined Benefit Plan Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Changes in the fair value of the Company's Level 3 investments
Plan assets at beginning of year	$ 20,970	$ 22,052
Actual return on plan assets	1,930	622
Plan assets at end of year	21,609	20,970
Level 3
Changes in the fair value of the Company's Level 3 investments
Plan assets at beginning of year	2,005	1,815
Actual return on plan assets	128	190
Purchases and sales of plan assets, net	(35)	0
Plan assets at end of year	$ 2,098	$ 2,005