Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Written premiums:
Direct		$ 16.5	$ 24.1	$ 37.3
Assumed		0.0	14.0	0.0
Gross written premiums		16.5	38.1	37.3
Earned premiums:
Direct		7.4	15.8	21.4
Assumed		0.5	8.5	1.9
Gross earned premiums		7.9	24.3	23.3
Net earned premiums	$ 581.5	506.8	1,244.4	1,070.6
Loss and loss adjustment expenses:
Net incurred loss and loss adjustment expenses	296.9	265.8	666.6	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	239.0	189.4	863.4	727.1
Assumed	134.6	61.8	1,079.3	939.6
Gross written premiums	373.6	251.2	1,942.7	1,666.7
Ceded	(34.2)	(20.2)	(502.5)	(360.3)
Net written premiums	339.4	231.0	1,440.2	1,306.4
Earned premiums:
Direct	317.6	273.1	731.5	624.1
Assumed	432.6	370.0	805.3	684.6
Gross earned premiums	750.2	643.1	1,536.8	1,308.7
Ceded	(198.0)	(144.2)	(363.5)	(261.4)
Net earned premiums	552.2	498.9	1,173.3	1,047.3
Loss and loss adjustment expenses:
Gross	350.7	269.3	852.7	627.5
Ceded	(62.4)	(3.5)	(208.7)	(46.4)
Net incurred loss and loss adjustment expenses	288.3	265.8	644.0	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Written premiums:
Ceded	8.8	6.0	82.0	108.4
Earned premiums:
Ceded	45.0	60.6	62.8	75.4
Loss and loss adjustment expenses:
Ceded	$ 13.2	$ 7.3	$ 13.6	$ 7.9