Revenue - Disaggregation of Revenues By Performance Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of revenue by performance obligation
Total operating revenue	$ 2,633,977	$ 2,536,489	$ 2,477,298
Management fee revenue - policy issuance and renewal services
Disaggregation of revenue by performance obligation
Total operating revenue	1,913,166	1,841,794	1,810,457
Management fee revenue - policy issuance and renewal services | Transferred at point in time
Disaggregation of revenue by performance obligation
Total operating revenue	1,913,166	1,841,794	1,810,457
Management fee revenue - administrative services
Disaggregation of revenue by performance obligation
Total operating revenue	58,286	59,463	57,204
Management fee revenue - administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Total operating revenue	58,286	59,463	57,204
Administrative services reimbursement revenue
Disaggregation of revenue by performance obligation
Total operating revenue	638,483	609,435	582,010
Administrative services reimbursement revenue | Transferred over time
Disaggregation of revenue by performance obligation
Total operating revenue	638,483	609,435	582,010
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Total operating revenue	$ 696,769	$ 668,898	$ 639,214