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Related Party Transactions (Details) - USD ($) $ in Thousands	3 Months Ended	5 Months Ended	12 Months Ended			15 Months Ended
	Oct. 31, 2020	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Related Party Transaction [Line Items]
Sublease rental income			$ 11,329	$ 8,965	$ 3,671
Stagwell Subsidiary [Member] | Services Provided By Subisidiary [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction						$ 2,000
Due to Related Parties			1,200			1,200
Stagwell Subsidiary [Member] | Media Planning, Buying and Reporting [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction			56,700
Due from Related Parties			110			110
Stagwell Subsidiary [Member] | Development of Advertising Technology [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction			483
Due to Related Parties			2			2
Stagwell Subsidiary [Member] | Audience and Brand Research, Concept Testing and Landscape [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	$ 145
Due to Related Parties			63			63
Stagwell Subsidiary [Member] | Event Management Services [Member]
Related Party Transaction [Line Items]
Due from Related Parties			$ 67			$ 67
Forecast | Stagwell Subsidiary [Member] | Event Management Services [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction		$ 457

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