Pension Plans - Change in Benefit Obligation And Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	$ 69,879
Actual return on plan assets	(3,000)	$ 3,500	$ 1,000
Employer contributions	2,600	2,600	2,500
Plan assets at fair value—end of year	72,063	69,879
Unrecognized actuarial loss	(37,200)	(34,900)
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation, beginning of year	94,983	87,864
Service cost	1,426	1,372	1,535
Interest cost	527	1,199	1,260
Actuarial (gain) loss	(2,835)	12,059
Foreign exchange impact	6,452	(4,674)
Participant contributions	86	154
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Benefits paid	(2,234)	(1,836)
Curtailments	(573)	0
Settlements	0	(1,155)
Projected benefit obligation, end of year	97,832	94,983	87,864
Plan assets at fair value, beginning of year	69,879	70,141
Actual return on plan assets	(2,990)	3,512
Employer contributions	2,622	2,576
Participant contributions	86	154
Foreign exchange impact	4,700	(3,513)
Settlements	0	(1,155)
Benefits paid	(2,234)	(1,836)
Plan assets at fair value—end of year	72,063	69,879	$ 70,141
Underfunded status	(25,769)	(25,104)
Accumulated benefit obligation, end of year	96,270	92,280
Non-current liability	(25,437)	(24,868)
Liability, Defined Benefit Plan, Current	(332)	(236)
Unrecognized actuarial loss	$ 37,175	$ 34,920