SCHEDULE VI - SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - Ark/WM Outrigger - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Year	$ 706.9	$ 588.1	$ 336.3
Prior Year	19.9	(51.7)	(21.5)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	145.3	127.2	100.8
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,605.1	1,296.5	894.7
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	743.6	623.2	495.9
Earned Premiums	1,409.7	1,043.4	637.3
Net investment income	61.4	16.3	2.9
Current Year	706.9	588.1	336.3
Prior Year	19.9	(51.7)	(21.5)
Amortization of deferred policy acquisition costs	273.1	238.3	111.3
Paid Claims and Claims Adjustment Expenses	407.9	257.5	105.5
Premiums written	$ 1,520.9	$ 1,195.2	$ 859.1