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Business acquisitions - Enquero inc. - Narrative (Details) - USD ($) $ in Thousands		9 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Payment for business acquisitions, net of cash acquired		$ 6,613	$ 0
Goodwill	$ 1,695,688	$ 1,685,684		$ 1,574,466
Enquero Inc.
Business Acquisition [Line Items]
Ownership percentage acquired (in percentage)	100.00%
Purchase consideration	$ 148,797
Payment for business acquisitions, net of cash acquired	137,166
Cash and cash equivalents	11,631
Cash consideration to acquired certain assets and assumed certain liabilities	$ 141,938
Acquired intangible assets, weighted average amortization period	4 years
Goodwill	$ 86,561
Acquisition related cost	1,590
Acquired assets	32,759
Liabilities assumed	17,113
Recognized net deferred tax liability	13,310
Indemnification assets	5,848
Enquero Inc. \| BCMI
Business Acquisition [Line Items]
Goodwill	2,556
Enquero Inc. \| CGRLH
Business Acquisition [Line Items]
Goodwill	22,211
Enquero Inc. \| HMS
Business Acquisition [Line Items]
Goodwill	61,794
Enquero Inc. \| Customer-related intangible assets
Business Acquisition [Line Items]
Acquired intangibles	49,000
Enquero Inc. \| Marketing-related intangible assets
Business Acquisition [Line Items]
Acquired intangibles	9,500
Enquero Inc. \| Technology-related intangible assets
Business Acquisition [Line Items]
Acquired intangibles	$ 1,400