Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Contingency [Line Items]
Net deferred tax benefit		$ 68.0
Non-U.S. pre-tax income (loss)		282.7	$ 450.0	$ (94.4)
Net income tax payments to US		34.3	$ 42.7	$ 10.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment
Income Tax Contingency [Line Items]
Net deferred tax benefit		51.0
Financial Guarantee (HG Global) | HG Global
Income Tax Contingency [Line Items]
Net deferred tax benefit		17.0
Deferred tax liabilities		5.0
Tax benefit	$ 5.0
Domestic Tax Jurisdiction
Income Tax Contingency [Line Items]
Separate return limitation year operating loss carryforwards		7.8
Separate return limitation year internal revenue		0.7
Foreign Tax Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carryforwards		$ 2.8