SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	$ 165.2	$ 145.3	$ 127.2
Future Policy Benefits, Losses, Claims and Loss Expenses	2,127.5	1,605.1	1,296.5
Unearned Premiums	853.3	743.6	623.2
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premiums Earned	1,587.8	1,409.7	1,043.4
Net Investment Income	90.7	61.4	16.3
Benefits, Claims, Losses and Settlement Expenses	855.8	726.8	536.4
Amortization of Deferred Acquisition Costs	294.1	273.1	238.3
Other Operating Expenses	136.1	113.6	78.7
Premiums Written	1,679.1	1,520.9	1,195.2
Financial Guarantee (HG Global)
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	86.6	40.1	36.0
Future Policy Benefits, Losses, Claims and Loss Expenses	0.0	0.0	0.0
Unearned Premiums	297.3	325.8	298.3
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premiums Earned	31.7	31.2	33.3
Net Investment Income	32.2	31.7	21.5
Benefits, Claims, Losses and Settlement Expenses	0.0	0.0	0.0
Amortization of Deferred Acquisition Costs	8.2	8.6	11.2
Other Operating Expenses	0.2	0.4	0.4
Premiums Written	56.0	$ 58.6	$ 65.1
Bamboo Captive
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	10.2
Future Policy Benefits, Losses, Claims and Loss Expenses	17.8
Unearned Premiums	31.5
Other Policy Claims and Benefits Payable	0.0
Premiums Earned	39.4
Net Investment Income	1.4
Benefits, Claims, Losses and Settlement Expenses	20.6
Amortization of Deferred Acquisition Costs	14.1
Other Operating Expenses	0.0
Premiums Written	50.0
Bamboo CRV
SUPPLEMENTARY INSURANCE INFORMATION
Deferred Acquisition Costs	5.8
Future Policy Benefits, Losses, Claims and Loss Expenses	12.1
Unearned Premiums	29.0
Other Policy Claims and Benefits Payable	0.0
Premiums Earned	32.7
Net Investment Income	0.4
Benefits, Claims, Losses and Settlement Expenses	12.1
Amortization of Deferred Acquisition Costs	12.1
Other Operating Expenses	0.0
Premiums Written	$ 61.7