Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Written premiums:
Direct	$ 24.1	$ 58.6	$ 63.8
Assumed	31.9	0.0	1.3
Gross written premiums	56.0	58.6	65.1
Earned premiums:
Direct	15.8	28.7	28.6
Assumed	15.9	2.5	4.7
Gross earned premiums	31.7	31.2	33.3
Net earned premiums	1,691.6	1,440.9	1,076.7
Loss and LAE:
Net incurred loss and LAE	888.5	726.8	536.4
Ark Insurance Holdings Limited/Outrigger
Written premiums:
Direct	1,101.5	931.9	760.4
Assumed	1,105.5	966.5	691.6
Gross written premiums	2,207.0	1,898.4	1,452.0
Ceded	(527.9)	(377.5)	(256.8)
Net written premiums	1,679.1	1,520.9	1,195.2
Earned premiums:
Direct	1,004.2	848.1	655.5
Assumed	1,088.2	925.6	668.7
Gross earned premiums	2,092.4	1,773.7	1,324.2
Ceded	(504.6)	(364.0)	(280.8)
Net earned premiums	1,587.8	1,409.7	1,043.4
Loss and LAE:
Gross	1,056.7	782.5	814.9
Ceded	(200.9)	(55.7)	(278.5)
Net incurred loss and LAE	855.8	726.8	$ 536.4
Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment
Written premiums:
Ceded	86.5	(110.0)
Earned premiums:
Ceded	88.0	104.3
Loss and LAE:
Ceded	$ 29.9	$ 15.6