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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2026 USD ($) fund shares	Jun. 30, 2026 USD ($) fund infrastructureFund $ / shares shares	Dec. 31, 2025 USD ($) $ / shares shares
Concentration Risk [Line Items]
Investment portfolio duration		2 years 9 months 18 days	2 years 6 months
Weighted average portfolio period, excluding short-term investment		2 years 1 month 6 days	1 year 6 months
Other long-term investments	$ 3,316.2	$ 3,316.2	$ 2,958.5
Total investments	8,134.4	8,134.4	8,323.9
Unfunded Commitments	91.0	91.0	65.8
Private equity funds
Concentration Risk [Line Items]
Other long-term investments	$ 248.2	$ 248.2
Number of investments \| fund	18	18
Hedge funds
Concentration Risk [Line Items]
Number of investments \| fund	2	2
Unfunded Commitments	$ 0.0	$ 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	$ 313.4	$ 313.4	308.5
Written notice period	15 days	15 days
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	$ 196.0	$ 196.0	180.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments	52.2	$ 52.2	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]
Unfunded Commitments	300.0	$ 300.0
Number of infrastructure funds \| infrastructureFund		3
Prefunded commitments	$ 100.0
Written notice period	90 days	90 days
Infrastructure Fund \| Minimum
Concentration Risk [Line Items]
Lock up period		0 years
Infrastructure Fund \| Maximum
Concentration Risk [Line Items]
Lock up period		4 years
MediaAlpha
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) \| $ / shares		$ 7.50
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	$ 337.7	$ 337.7	347.0
Fair value measured on a recurring basis \| MediaAlpha
Concentration Risk [Line Items]
Other long-term investments	224.5	224.5	231.2
Fair value measured on a recurring basis \| Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	337.7	337.7	347.0
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	196.0	196.0	180.4
Lloyds Bank
Concentration Risk [Line Items]
Total investments	517.9	517.9	361.6
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments	326.0	326.0	257.2
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value	122.6	122.6	245.7
Distinguished Transaction
Concentration Risk [Line Items]
Pledged collateral	$ 67.9	$ 67.9	$ 56.8
MediaAlpha
Concentration Risk [Line Items]
Investment owned (in shares) \| shares	17.9	17.9	17.9
Ownership interest (as a percent)	29.10%	29.10%	27.70%
Purchase price (in dollars per share) \| $ / shares		$ 12.57	$ 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	$ 170.0	$ 170.0	$ 110.6