Revenue - Disaggregation of Revenue by Performance Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of revenue by performance obligation
Revenue	$ 3,268,940	$ 2,840,124	$ 2,633,977
Management fee revenue - policy issuance and renewal services
Disaggregation of revenue by performance obligation
Revenue	2,442,073	2,087,846	1,913,166
Management fee revenue - policy issuance and renewal services | Transferred at point in time
Disaggregation of revenue by performance obligation
Revenue	2,442,073	2,087,846	1,913,166
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Revenue	800,808	726,591	696,769
Management fee revenue - administrative services
Disaggregation of revenue by performance obligation
Revenue	63,669	58,323	58,286
Management fee revenue - administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Revenue	63,669	58,323	58,286
Administrative services reimbursement revenue
Disaggregation of revenue by performance obligation
Revenue	737,139	668,268	638,483
Administrative services reimbursement revenue | Transferred over time
Disaggregation of revenue by performance obligation
Revenue	$ 737,139	$ 668,268	$ 638,483