Overview and Basis of Presentation - Schedule of Total Advertising Costs Included in Direct Salaries and Related Costs in Condensed Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Direct Salaries and Related Costs [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Customer-acquisition advertising costs	$ 21,297	$ 9,826	$ 40,862	$ 20,008