Fair Value Measurements (Rollforward of Earn-outs) (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Option and purchase right		$ 506		$ 0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Payments		(187)
Earn-outs related to acquisitions		91		1,545
Fair value increase		(1,267)
Option and purchase right		(7,673)
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Other Current Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Payments		(187)
Earn-outs related to acquisitions	[1]	91		611
Fair value increase	[1]	(333)
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Other Noncurrent Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Payments		0
Earn-outs related to acquisitions	[2]	0		$ 934
Fair value increase	[2]	(934)
Frontier [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Remaining percentage to be acquired			40.00%
Frontier [Member] | Other Current Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Option and purchase right		$ 506
Frontier [Member] | Other Noncurrent Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Option and purchase right			$ 7,167

[1] Included in Accrued expenses and other liabilities on the Consolidated Balance Sheets.

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