Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Written premiums:
Direct				$ 24.1
Assumed				14.0
Gross written premiums				38.1
Earned premiums:
Direct				15.8
Assumed				8.5
Gross earned premiums				24.3
Net earned premiums	$ 557.8	$ 581.5	$ 1,328.0	1,244.4
Loss and LAE:
Net incurred loss and LAE	258.5	296.9	686.8	666.6
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Written premiums:
Direct	240.0	239.0	1,013.4	863.4
Assumed	126.4	134.6	1,275.8	1,079.3
Gross written premiums	366.4	373.6	2,289.2	1,942.7
Ceded	(79.4)	(34.2)	(695.9)	(502.5)
Net written premiums	287.0	339.4	1,593.3	1,440.2
Earned premiums:
Direct	334.3	317.6	840.7	731.5
Assumed	451.8	432.6	874.9	805.3
Gross earned premiums	786.1	750.2	1,715.6	1,536.8
Ceded	(243.9)	(198.0)	(451.2)	(363.5)
Net earned premiums	542.2	552.2	1,264.4	1,173.3
Loss and LAE:
Gross	332.0	350.7	852.2	852.7
Ceded	(77.4)	(62.4)	(200.1)	(208.7)
Net incurred loss and LAE	254.6	288.3	652.1	644.0
Effects of Reinsurance Footnotes [Abstract]
Ceded premiums written	79.4	34.2	695.9	502.5
Ceded premiums earned	243.9	198.0	451.2	363.5
Ceded	77.4	62.4	200.1	208.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Written premiums:
Ceded	(3.6)	(8.8)	(83.7)	(82.0)
Earned premiums:
Ceded	(43.6)	(45.0)	(62.7)	(62.8)
Loss and LAE:
Ceded	2.2	(13.2)	24.1	13.6
Effects of Reinsurance Footnotes [Abstract]
Ceded premiums written	3.6	8.8	83.7	82.0
Ceded premiums earned	43.6	45.0	62.7	62.8
Ceded	$ (2.2)	$ 13.2	$ (24.1)	$ (13.6)