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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, 2024 USD ($) $ / shares shares	Jun. 30, 2025 USD ($) fund $ / shares shares	Dec. 31, 2024 USD ($) $ / shares
Concentration Risk [Line Items]
Other long-term investments		$ 2,307.1	$ 2,150.2
Total investments		6,798.6	6,477.6
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 247.0
Number of investments \| fund		17
Hedge funds
Concentration Risk [Line Items]
Number of investments \| fund		2
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		$ 270.9	264.7
Written notice period		15 days
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		$ 159.0	149.9
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 73.6	74.0
ILS funds \| Minimum
Concentration Risk [Line Items]
Notice period		30 days
ILS funds \| Maximum
Concentration Risk [Line Items]
Notice period		90 days
IPO \| MediaAlpha
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) \| $ / shares		$ 7.00
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		$ 306.8	236.7
Fair value measured on a recurring basis \| MediaAlpha
Concentration Risk [Line Items]
Other long-term investments		195.5	201.6
Fair value measured on a recurring basis \| Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		306.8	236.7
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		159.0	149.9
Lloyds Bank
Concentration Risk [Line Items]
Total investments		357.8	361.5
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments		243.1	226.5
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value		220.2	203.7
Bamboo CRV
Concentration Risk [Line Items]
Restricted cash		41.9	32.2
Pledged collateral		$ 41.3	$ 46.7
MediaAlpha
Concentration Risk [Line Items]
Shares purchased (in shares) \| shares	7.6
Purchase price (in dollars per share) \| $ / shares	$ 19.00	$ 10.95	$ 11.29
Purchase price net of underwriting fees (in dollars per share) \| $ / shares	$ 18.24
Investment owned (in shares) \| shares		17.9
Ownership interest (as a percent)		26.30%	26.60%
Fully diluted ownership interest (as a percent)		24.30%
Increase in investment owned (in dollars per share) \| $ / shares		$ 1.00
MediaAlpha \| Secondary Offering
Concentration Risk [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares	5.0
Proceeds from sale of business	$ 91.2
Largest Investment \| Investments \| Private Equity and Hedge Fund Concentration Risk
Concentration Risk [Line Items]
Other long-term investments		$ 84.1	$ 59.2