SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Year	$ 914.4	$ 706.9	$ 588.1
Prior Year	(58.6)	19.9	(51.7)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | P&C Insurance and Reinsurance (Ark/WM Outrigger)
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	165.2	145.3	127.2
Reserves for Unpaid Claims and Claims Adjustment Expenses	2,127.5	1,605.1	1,296.5
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	853.3	743.6	623.2
Earned Premiums	1,587.8	1,409.7	1,043.4
Net investment income	90.7	61.4	16.3
Current Year	914.4	706.9	588.1
Prior Year	(58.6)	19.9	(51.7)
Amortization of deferred policy acquisition costs	294.1	273.1	238.3
Paid Claims and Claims Adjustment Expenses	416.7	407.9	257.5
Net Premiums written	1,679.1	1,520.9	1,195.2
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | Bamboo Captive
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	10.2	0.0	0.0
Reserves for Unpaid Claims and Claims Adjustment Expenses	17.8	0.0	0.0
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	31.5	0.0	0.0
Earned Premiums	39.4	0.0	0.0
Net investment income	1.4	0.0	0.0
Current Year	17.3	0.0	0.0
Prior Year	3.3	0.0	0.0
Amortization of deferred policy acquisition costs	14.1	0.0	0.0
Paid Claims and Claims Adjustment Expenses	12.0	0.0	0.0
Net Premiums written	50.0	0.0	0.0
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | Bamboo CRV
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	5.8	0.0	0.0
Reserves for Unpaid Claims and Claims Adjustment Expenses	12.1	0.0	0.0
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	29.0	0.0	0.0
Earned Premiums	32.7	0.0	0.0
Net investment income	0.4	0.0	0.0
Current Year	12.1	0.0	0.0
Prior Year	0.0	0.0	0.0
Amortization of deferred policy acquisition costs	12.1	0.0	0.0
Paid Claims and Claims Adjustment Expenses	0.0	0.0	0.0
Net Premiums written	$ 61.7	$ 0.0	$ 0.0