Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Earned premiums:
Net earned premiums	$ 301.0	$ 227.8	$ 563.8	$ 430.6
Ark
Written premiums:
Direct	291.5	214.3	537.7	445.2
Assumed	314.6	189.6	877.8	591.8
Gross written premiums	606.1	403.9	1,415.5	1,037.0
Ceded	(144.9)	(133.7)	(340.1)	(223.0)
Net written premiums	461.2	270.2	1,075.4	814.0
Earned premiums:
Direct	200.6	160.7	351.0	299.4
Assumed	152.5	105.7	314.6	212.3
Gross earned premiums	353.1	266.4	665.6	511.7
Ceded	(59.8)	(49.1)	(117.2)	(100.0)
Net earned premiums	293.3	217.3	548.4	411.7
Losses and LAE:
Policyholder Benefits and Claims Incurred, Assumed	195.8	138.4	358.2	321.5
Policyholder Benefits and Claims Incurred, Ceded	(28.3)	(17.9)	(42.9)	(79.0)
Policyholder Benefits and Claims Incurred, Net, Total	167.5	$ 120.5	315.3	$ 242.5
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Ceded	(58.3)		(102.4)
Earned premiums:
Ceded	(9.6)		(14.8)
Losses and LAE:
Policyholder Benefits and Claims Incurred, Ceded	$ (0.4)		$ (0.6)