Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Earned premiums:
Net earned premiums	$ 319.0	$ 262.8
Loss and loss adjustment expenses:
Net incurred loss and loss adjustment expenses	185.8	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	285.0	246.2
Assumed	587.1	563.2
Gross written premiums	872.1	809.4
Ceded	(274.1)	(195.2)
Net written premiums	598.0	614.2
Earned premiums:
Direct	194.4	150.4
Assumed	192.4	162.1
Gross earned premiums	386.8	312.5
Ceded	(84.0)	(57.4)
Net earned premiums	302.8	255.1
Loss and loss adjustment expenses:
Gross	253.3	162.4
Ceded	(73.3)	(14.6)
Net incurred loss and loss adjustment expenses	180.0	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ark
Written premiums:
Ceded	(34.3)	44.1
Earned premiums:
Ceded	(10.3)	5.2
Loss and loss adjustment expenses:
Ceded	$ (0.7)	$ 0.2

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