Pension Plans - Components of Net Periodic Pension Cost (Details) - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Interest cost of projected benefit obligation	$ 527	$ 1,199	$ 1,260
Service cost	1,426	1,372	1,535
Expected return on plan assets	(3,878)	(3,728)	(4,180)
Amortization of prior service cost	(5)	(5)	(5)
Recognized actuarial loss	3,854	2,390	2,293
Settlement loss	0	(30)	9
Net periodic pension cost	$ 1,924	$ 1,198	$ 912