Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020	Oct. 01, 2019
Fair Value, Measurements, Recurring | Quoted Market Prices in Active Markets (Level 1)
Deferred compensation plan assets
Trust Assets		$ 55
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets:
Foreign currency forward contracts	$ 173	140
Deferred compensation plan assets
Insurance	20,777	19,804
Liabilities:
Pension plan	(49,599)	(47,389)
Deferred compensation plan liabilities	(22,833)	(21,439)
Long‑term debt	(885,971)	(872,441)
Interest Rate Derivative Liabilities, at Fair Value	(3,703)	(4,669)
Foreign currency forward contracts	(53)	(47)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities:
Earn-outs related to acquisitions	(295)	(295)
Option and purchase right	(7,739)	(7,739)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Other Current Liabilities
Liabilities:
Earn-outs related to acquisitions	(295)	(295)
Option and purchase right	7,739	7,739
Fair Value, Measurements, Recurring | Cash | Quoted Market Prices in Active Markets (Level 1)
Assets:
Defined benefit plan, fair value of plan assets	769	15,061
Fair Value, Measurements, Recurring | US Government Agencies Debt Securities
Assets:
Plan assets at net asset value	3,034	4,924
Fair Value, Measurements, Recurring | Liability Driven Investment
Assets:
Plan assets at net asset value	6,126	3,604
Fair Value, Measurements, Recurring | Guernsey Unit Trust
Assets:
Plan assets at net asset value	2,100	1,881
Fair Value, Measurements, Recurring | Global Absolute Return
Assets:
Plan assets at net asset value	2,223	$ 2,060
Fair Value, Measurements, Recurring | Hedge Funds, Multi-strategy
Assets:
Plan assets at net asset value	$ 14,895
Frontier [Member]
Liabilities:
Remaining percentage to be acquired			40.00%