Acquisitions - Summary of Purchase Price Allocation (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jul. 01, 2021	Dec. 31, 2020	Oct. 13, 2020
Business Acquisition [Line Items]
Goodwill	$ 68,728		$ 58,043
Cisneros Interactive
Business Acquisition [Line Items]
Cash				$ 8,700
Accounts receivable				50,500
Other assets				8,300
Intangible assets subject to amortization				41,700
Goodwill				9,900
Current liabilities				(48,100)
Deferred tax				(10,300)
Redeemable noncontrolling interest				$ 30,800
MediaDonuts
Business Acquisition [Line Items]
Cash		$ 4,300
Accounts receivable		9,900
Other assets		1,800
Intangible assets subject to amortization		22,800
Goodwill		13,100
Current liabilities		(10,100)
Deferred tax		(3,900)
Debt		$ (1,700)