Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earned premiums:
Net earned premiums	$ 1,440.9	$ 1,076.7	$ 664.2
Losses and LAE:
Net incurred losses and LAE	726.8	536.4	314.8
Ark Insurance Holdings Limited/Outrigger
Written premiums:
Direct	931.9	760.4	619.5
Assumed	966.5	691.6	439.2
Gross written premiums	1,898.4	1,452.0	1,058.7
Ceded	(377.5)	(256.8)	(199.6)
Net written premiums	1,520.9	1,195.2	859.1
Earned premiums:
Direct	848.1	655.5	556.0
Assumed	925.6	668.7	330.4
Gross earned premiums	1,773.7	1,324.2	886.4
Ceded	(364.0)	(280.8)	(249.1)
Net earned premiums	1,409.7	1,043.4	637.3
Losses and LAE:
Gross	782.5	814.9	442.9
Ceded	(55.7)	(278.5)	(128.1)
Net incurred losses and LAE	726.8	$ 536.4	$ 314.8
Ark Insurance Holdings Limited/Outrigger \| Ark
Written premiums:
Ceded	110.0
Earned premiums:
Ceded	104.3
Losses and LAE:
Ceded	$ 15.6

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