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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2025 USD ($) fund $ / shares shares	Jun. 30, 2024 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) $ / shares
Concentration Risk [Line Items]
Other long-term investments	$ 2,287.9		$ 2,150.2
Total investments	6,551.3		6,477.6
Private equity funds
Concentration Risk [Line Items]
Other long-term investments	$ 240.6
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	$ 264.9		264.7
Written notice period	15 days
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	$ 155.9		149.9
ILS funds
Concentration Risk [Line Items]
Other long-term investments	$ 71.3		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	$ 256.0		236.7
Fair value measured on a recurring basis \| MediaAlpha
Concentration Risk [Line Items]
Other long-term investments	165.0		201.6
Fair value measured on a recurring basis \| Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	256.0		236.7
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	155.9		149.9
Lloyds Bank
Concentration Risk [Line Items]
Total investments	348.2		361.5
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments	250.3		226.5
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value	208.7		203.7
Bamboo CRV
Concentration Risk [Line Items]
Restricted cash	41.3		32.2
Pledged collateral	$ 42.7		$ 46.7
MediaAlpha
Concentration Risk [Line Items]
Shares purchased (in shares) \| shares		7.6
Purchase price (in dollars per share) \| $ / shares	$ 9.24	$ 19.00	$ 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)	26.50%		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	$ 80.6		$ 59.2