Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earned premiums:
Net earned premiums	$ 506.8	$ 353.2	$ 1,070.6	$ 783.8
Losses and LAE:
Net incurred losses and LAE	265.8	213.7	581.1	456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	189.4	118.6	727.1	563.8
Assumed	61.8	96.9	939.6	688.7
Gross written premiums	251.2	215.5	1,666.7	1,252.5
Ceded	(20.2)	(22.9)	(360.3)	(246.0)
Net written premiums	231.0	192.6	1,306.4	1,006.5
Earned premiums:
Direct	273.1	193.5	624.1	492.9
Assumed	370.0	261.4	684.6	473.7
Gross earned premiums	643.1	454.9	1,308.7	966.6
Ceded	(144.2)	(108.8)	(261.4)	(208.8)
Net earned premiums	498.9	346.1	1,047.3	757.8
Losses and LAE:
Gross	269.3	391.4	627.5	712.9
Ceded	(3.5)	(177.7)	(46.4)	(256.7)
Net incurred losses and LAE	265.8	$ 213.7	581.1	$ 456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Written premiums:
Ceded	(6.0)		(108.4)
Earned premiums:
Ceded	(60.6)		(75.4)
Losses and LAE:
Ceded	$ (7.3)		$ (7.9)