Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Earned premiums:
Net earned premiums	$ 343.9	$ 301.0	$ 662.9	$ 563.8
Loss and loss adjustment expenses:
Net incurred loss and loss adjustment expenses	183.9	167.5	369.7	315.3
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	339.4	291.5	624.4	537.7
Assumed	357.6	314.6	944.7	877.8
Gross written premiums	697.0	606.1	1,569.1	1,415.5
Ceded	(194.2)	(144.9)	(468.3)	(340.1)
Net written premiums	502.8	461.2	1,100.8	1,075.4
Earned premiums:
Direct	219.5	200.6	413.9	351.0
Assumed	180.3	152.5	372.7	314.6
Gross earned premiums	399.8	353.1	786.6	665.6
Ceded	(81.5)	(59.8)	(165.5)	(117.2)
Net earned premiums	318.3	293.3	621.1	548.4
Loss and loss adjustment expenses:
Gross	248.7	195.8	502.0	358.2
Ceded	(73.0)	(28.3)	(146.3)	(42.9)
Net incurred loss and loss adjustment expenses	175.7	167.5	355.7	315.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ark
Written premiums:
Ceded	38.9	58.3	(73.2)	102.4
Earned premiums:
Ceded	7.5	9.6	(17.8)	14.8
Loss and loss adjustment expenses:
Ceded	$ 0.3	$ 0.4	$ (0.4)	$ 0.6

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