Revenue - Disaggregation of Revenues By Performance Obligations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of revenue by performance obligation
Operating revenue	$ 596,740	$ 638,742	$ 647,710	$ 594,106	$ 576,468	$ 612,126	$ 621,458	$ 572,160	$ 2,477,298	$ 2,382,212	$ 1,691,774
Management fee revenue - policy issuance and renewal services, net
Disaggregation of revenue by performance obligation
Operating revenue									1,810,457	1,719,567	1,662,625
Management fee revenue - policy issuance and renewal services, net | Transferred at point in time
Disaggregation of revenue by performance obligation
Operating revenue									1,810,457	1,719,567	1,662,625
Management fee revenue - administrative services, net
Disaggregation of revenue by performance obligation
Operating revenue									57,204	53,632	0
Management fee revenue - administrative services, net | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue									57,204	53,632	0
Administrative services reimbursement revenue
Disaggregation of revenue by performance obligation
Operating revenue									582,010	580,336	0
Administrative services reimbursement revenue | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue									582,010	580,336	0
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue									$ 639,214	$ 633,968	$ 0