Revenue - Disaggregation of Revenues By Performance Obligations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of revenue by performance obligation
Operating revenue	$ 647,710	$ 621,458	$ 1,241,816	$ 1,193,618
Management fee revenue - policy issuance and renewal services, net
Disaggregation of revenue by performance obligation
Operating revenue	480,513	454,572	911,496	860,550
Management fee revenue - policy issuance and renewal services, net | Transferred at point in time
Disaggregation of revenue by performance obligation
Operating revenue	480,513	454,572	911,496	860,550
Management fee revenue - administrative services, net
Disaggregation of revenue by performance obligation
Operating revenue	14,195	13,299	28,146	26,373
Management fee revenue - administrative services, net | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	14,195	13,299	28,146	26,373
Administrative services reimbursement revenue
Disaggregation of revenue by performance obligation
Operating revenue	146,095	146,507	288,575	292,470
Administrative services reimbursement revenue | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	146,095	146,507	288,575	292,470
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	$ 160,290	$ 159,806	$ 316,721	$ 318,843