Revenue - Disaggregation of Revenues By Performance Obligations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of revenue by performance obligation
Operating revenue	$ 638,742	$ 612,126	$ 1,880,558	$ 1,805,744
Management fee revenue - policy issuance and renewal services, net
Disaggregation of revenue by performance obligation
Operating revenue	474,427	451,361	1,385,923	1,311,911
Management fee revenue - policy issuance and renewal services, net | Transferred at point in time
Disaggregation of revenue by performance obligation
Operating revenue	474,427	451,361	1,385,923	1,311,911
Management fee revenue - administrative services, net
Disaggregation of revenue by performance obligation
Operating revenue	14,430	13,521	42,576	39,894
Management fee revenue - administrative services, net | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	14,430	13,521	42,576	39,894
Administrative services reimbursement revenue
Disaggregation of revenue by performance obligation
Operating revenue	142,730	140,172	431,305	432,642
Administrative services reimbursement revenue | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	142,730	140,172	431,305	432,642
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	$ 157,160	$ 153,693	$ 473,881	$ 472,536