Summary of Significant Accounting Policies - Revenue - Contract Fulfillment Costs (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Intake Costs asset
Capitalized Contract Cost [Line Items]
GROSS CARRYING AMOUNT	$ 63,721	$ 41,224
ACCUMULATED AMORTIZATION	(33,352)	(23,579)
NET CARRYING AMOUNT	30,369	17,645
Commissions asset
Capitalized Contract Cost [Line Items]
GROSS CARRYING AMOUNT	91,069	68,008
ACCUMULATED AMORTIZATION	(38,787)	(27,178)
NET CARRYING AMOUNT	$ 52,282	$ 40,830