Fair Value Measurements (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019
Liabilities:
Option and purchase right		$ 506		$ 0
Fair Value, Measurements, Recurring | Quoted Market Prices in Active Markets (Level 1)
Deferred compensation plan assets
Trust Assets		90		16
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets:
Foreign currency forward contracts		148		278
Deferred compensation plan assets
Insurance		19,124		18,684
Liabilities:
Pension plan		(44,667)		(42,948)
Deferred compensation plan liabilities		(21,874)		(21,318)
Foreign currency forward contracts		(123)		(154)
Long‑term debt		(979,691)		(979,357)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities:
Earn-outs related to acquisitions		(91)		(1,545)
Option and purchase right		(7,673)
Fair Value, Measurements, Recurring | Cash | Quoted Market Prices in Active Markets (Level 1)
Assets:
Defined benefit plan, fair value of plan assets		15,054		14,607
Fair Value, Measurements, Recurring | US Government Agencies Debt Securities
Assets:
Plan assets at net asset value		2,034		4,703
Fair Value, Measurements, Recurring | Liability Driven Investment
Assets:
Plan assets at net asset value		5,379		3,261
Fair Value, Measurements, Recurring | Guernsey Unit Trust
Assets:
Plan assets at net asset value		1,072		997
Fair Value, Measurements, Recurring | Global Absolute Return
Assets:
Plan assets at net asset value		2,103		1,957
Other Noncurrent Liabilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities:
Earn-outs related to acquisitions	[1]	$ 0		$ (934)
Frontier [Member] | Other Noncurrent Liabilities
Liabilities:
Option and purchase right			$ 7,167

[1] Included in Other non‑current liabilities on the Consolidated Balance Sheets.