Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	$ 26,852	$ 19,442	$ 73,751	$ 56,368
Platform operations
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	1,639	1,507	5,459	3,894
Sales and marketing
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	6,916	5,036	18,549	13,094
Technology and development
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	7,911	7,146	24,345	18,579
General and administrative
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	$ 10,386	$ 5,753	$ 25,398	$ 20,801