Summary of Significant Accounting Policies - Contract Fulfillment Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Intake Costs
Contract With Customer, Asset And Liabilities [Line Items]
GROSS CARRYING AMOUNT	$ 71,540	$ 63,721
ACCUMULATED AMORTIZATION	(41,246)	(33,352)
NET CARRYING AMOUNT	30,294	30,369
Commissions
Contract With Customer, Asset And Liabilities [Line Items]
GROSS CARRYING AMOUNT	107,298	91,069
ACCUMULATED AMORTIZATION	(48,708)	(38,787)
NET CARRYING AMOUNT	$ 58,590	$ 52,282