Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of revenue by performance obligation
Operating revenue	$ 576,468	$ 612,126	$ 621,458	$ 572,160	$ 401,402	$ 442,492	$ 448,564	$ 399,316	$ 2,382,212	$ 1,691,774	$ 1,596,631
Management fee revenue, policy issuance and renewal services
Disaggregation of revenue by performance obligation
Gross management fee revenue recognized									1,721,309
Operating revenue									1,719,567	1,662,625	1,567,431
Management fee revenue, policy issuance and renewal services | Transferred at point in time
Disaggregation of revenue by performance obligation
Operating revenue									1,719,567	1,662,625	1,567,431
Management fee revenue, administrative services
Disaggregation of revenue by performance obligation
Gross management fee revenue recognized									53,694
Operating revenue									53,632
Management fee revenue, administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue									53,632	0	0
Administrative services reimbursements
Disaggregation of revenue by performance obligation
Operating revenue									580,336
Administrative services reimbursements | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue									580,336	0	0
Administrative services
Disaggregation of revenue by performance obligation
Operating revenue									633,968
Administrative services | Transferred over time
Disaggregation of revenue by performance obligation
Operating revenue									$ 633,968	$ 0	$ 0