Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Earned premiums:
Net earned premiums	$ 262.8	$ 202.8
Ark
Written premiums:
Direct	246.2	230.9
Assumed	563.2	402.2
Gross written premiums	809.4	633.1
Ceded	(195.2)	(89.3)
Net written premiums	614.2	543.8
Earned premiums:
Direct	150.4	138.7
Assumed	162.1	106.6
Gross earned premiums	312.5	245.3
Ceded	(57.4)	(50.9)
Net earned premiums	255.1	194.4
Losses and LAE:
Policyholder Benefits and Claims Incurred, Assumed	162.4	183.1
Policyholder Benefits and Claims Incurred, Ceded	(14.6)	(61.1)
Policyholder Benefits and Claims Incurred, Net, Total	147.8	$ 122.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Ceded	(44.1)
Earned premiums:
Ceded	(5.2)
Losses and LAE:
Policyholder Benefits and Claims Incurred, Ceded	$ (0.2)