Deferred Revenue of Membership Upgrade Sales and Deferred Commission Expense - Schedule of Change in Deferred Revenue and Deferred Commissions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Movement in Deferred Revenue
Deferred revenue - upfront payments from membership upgrade sales as of December 31,	$ 126,814	$ 116,363
Membership upgrade sales current period, gross	21,739	19,111
Revenue recognized from membership upgrade sales upfront payments	(9,675)	(8,660)	$ (7,811)
Net increase in deferred revenue - upfront payments from membership grade sales	12,064	10,451
Deferred revenue - upfront payments from membership upgrade sales as of December 31,	138,878	126,814	116,363
Movement in Deferred Costs
Deferred commission expense as of December 31	41,149	40,308
Deferred commission expense	4,995	4,508
Commission expense recognized	(3,673)	(3,667)
Net increase in deferred commission expense	1,322	841
Deferred commission expense as of December 31,	$ 42,471	$ 41,149	$ 40,308