Revenue - Disaggregation of Revenues By Performance Obligations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of revenue by performance obligation
Operating revenue	$ 594,106	$ 572,160
Management fee revenue - policy issuance and renewal services
Disaggregation of revenue by performance obligation
Operating revenue	430,983	405,978
Management fee revenue - policy issuance and renewal services | Transferred at point in time
Disaggregation of revenue by performance obligation
Operating revenue	430,983	405,978
Management fee revenue - administrative services
Disaggregation of revenue by performance obligation
Operating revenue	13,951	13,074
Management fee revenue - administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	13,951	13,074
Administrative services reimbursements
Disaggregation of revenue by performance obligation
Operating revenue	142,480	145,963
Administrative services reimbursements | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	142,480	145,963
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue	$ 156,431	$ 159,037