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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2026 USD ($) fund $ / shares shares	Dec. 31, 2025 USD ($) $ / shares shares	Mar. 31, 2025 USD ($)
Concentration Risk [Line Items]
Investment portfolio duration	2 years 7 months 6 days	2 years 6 months
Weighted average portfolio period, excluding short-term investment	1 year 8 months 12 days	1 year 6 months
Other long-term investments	$ 3,253.5	$ 2,958.5
Total investments	$ 8,353.4	8,323.9
MediaAlpha
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) \| $ / shares	$ 7.00
MediaAlpha \| Fair value measured on a recurring basis
Concentration Risk [Line Items]
Other long-term investments	$ 166.1	231.2
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	176.4	180.4
Lloyds Bank
Concentration Risk [Line Items]
Total investments	353.7	361.6
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments	253.6	257.2
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value	117.7	245.7
Distinguished Transaction
Concentration Risk [Line Items]
Pledged collateral	$ 56.0	$ 56.8
MediaAlpha
Concentration Risk [Line Items]
Investment owned (in shares) \| shares	17.9	17.9
Ownership interest (as a percent)	28.40%	27.70%
Purchase price (in dollars per share) \| $ / shares	$ 9.30	$ 12.95
Increase in investment owned (in dollars per share) \| $ / shares	$ 1.00
Largest Investment \| Investments \| Private Equity and Hedge Fund Concentration Risk
Concentration Risk [Line Items]
Other long-term investments	$ 162.6	$ 110.6
Private equity funds
Concentration Risk [Line Items]
Other long-term investments	$ 249.1
Number of investments \| fund	17
Unfunded Commitments	$ 61.0	65.8
Hedge funds
Concentration Risk [Line Items]
Number of investments \| fund	2
Unfunded Commitments	$ 0.0	0.0
Hedge funds \| Minimum
Concentration Risk [Line Items]
Notice period	45 days
Hedge funds \| Maximum
Concentration Risk [Line Items]
Notice period	90 days
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments	$ 307.8	308.5
Written notice period	15 days
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	$ 176.4	180.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments	$ 51.0	$ 50.1
ILS funds \| Minimum
Concentration Risk [Line Items]
Notice period	30 days
ILS funds \| Maximum
Concentration Risk [Line Items]
Notice period	90 days
Infrastructure Fund
Concentration Risk [Line Items]
Written notice period			90 days
Unfunded Commitments			$ 100.0
Lock up period	4 years