Revenue - Disaggregation of Revenues By Performance Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of revenue by performance obligation
Operating revenue	$ 2,536,489	$ 2,477,298	$ 2,382,212
Management fee revenue - policy issuance and renewal services, net
Disaggregation of revenue by performance obligation
Operating revenue	1,841,794	1,810,457	1,719,567
Management fee revenue - policy issuance and renewal services, net | Transferred at point in time
Disaggregation of revenue by performance obligation
Operating revenue	1,841,794	1,810,457	1,719,567
Management fee revenue - administrative services, net
Disaggregation of revenue by performance obligation
Operating revenue	59,463	57,204	53,632
Management fee revenue - administrative services, net | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	59,463	57,204	53,632
Administrative services reimbursement revenue
Disaggregation of revenue by performance obligation
Operating revenue	609,435	582,010	580,336
Administrative services reimbursement revenue | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	609,435	582,010	580,336
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	$ 668,898	$ 639,214	$ 633,968