Summary of Significant Accounting Policies - Revenue - Amortization Expense Associated with Commissions Asset and Intake Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Intake Costs asset
Capitalized Contract Cost [Line Items]
Amortization expense	$ 13,300	$ 10,144	$ 10,380
Commissions asset
Capitalized Contract Cost [Line Items]
Amortization expense	$ 24,052	$ 19,109	$ 13,838