Joint Venture - Additional Information (Detail) - Roblox China Holding Corp $ in Millions	Feb. 28, 2019 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	51.00%
Songhua River Investment Limited
Schedule of Equity Method Investments [Line Items]
Minority interest percentage in joint venture	49.00%
Songhua River Investment Limited
Schedule of Equity Method Investments [Line Items]
Contribution by non controlling interest to the joint venture	$ 50