Summary of Significant Accounting Policies - Contract Fulfillment Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Intake Costs
Contract With Customer, Asset And Liabilities [Line Items]
GROSS CARRYING AMOUNT	$ 70,263	$ 63,721
ACCUMULATED AMORTIZATION	(39,083)	(33,352)
NET CARRYING AMOUNT	31,180	30,369
Commissions
Contract With Customer, Asset And Liabilities [Line Items]
GROSS CARRYING AMOUNT	104,184	91,069
ACCUMULATED AMORTIZATION	(45,898)	(38,787)
NET CARRYING AMOUNT	$ 58,286	$ 52,282