Business Acquisitions (Narrative) (Details)		9 Months Ended		12 Months Ended
	Jun. 04, 2021 USD ($)	Sep. 30, 2022 USD ($) business	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 1,579,957,000		$ 1,531,455,000
Goodwill acquired during period		93,827,000
Increase (decrease) in goodwill from adjustment under purchase accounting		4,806,000
Increase to goodwill related to an assumed liability		$ 3,200,000
Recognition period		4 years
Series of Individually Immaterial Business Acquisitions
Business Acquisition [Line Items]
Number of businesses acquired | business		4
Fiscal 2022 Acquisitions
Business Acquisition [Line Items]
Revenue of acquiree since acquisition date		$ 19,600,000
Total consideration of transactions		121,700,000
Fair value of accounts receivable acquired		7,400,000
Amount expected to be uncollectable		0
Goodwill		93,827,000
Expected income tax deductible amount		1,200,000
Accounts receivable		7,433,000
Fiscal 2022 Acquisitions | Digital Media
Business Acquisition [Line Items]
Increase (decrease) in goodwill from adjustment under purchase accounting		4,900,000
Fiscal 2022 Acquisitions | Cybersecurity and Martech
Business Acquisition [Line Items]
Increase (decrease) in goodwill from adjustment under purchase accounting		$ (100,000)
Fiscal 2021 Acquisitions
Business Acquisition [Line Items]
Revenue of acquiree since acquisition date			$ 21,300,000
Total consideration of transactions			134,300,000
Goodwill			84,019,000
Accounts receivable			4,577,000
SEOmoz Acquisition
Business Acquisition [Line Items]
Revenue of acquiree since acquisition date			$ 14,400,000
Total consideration of transactions	$ 67,000,000
Amount expected to be uncollectable	300,000
Goodwill	41,192,000
Goodwill acquired during period				41,200,000
Expected income tax deductible amount				$ 0
Accounts receivable	3,278,000
Gross amounts due under contracts	$ 3,600,000