Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares	Sep. 30, 2025 USD ($) fund $ / shares shares	Dec. 31, 2024 USD ($) $ / shares
Concentration Risk [Line Items]
Other long-term investments		$ 2,589.2	$ 2,150.2
Total investments		6,998.6	6,417.6
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 246.1
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		$ 305.4	264.7
Written notice period		15 days
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		$ 170.8	149.9
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 49.8	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		$ 362.1	236.7
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments		203.2	201.6
Fair value measured on a recurring basis | Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		362.1	236.7
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		170.8	149.9
Lloyds Bank
Concentration Risk [Line Items]
Total investments		361.3	361.5
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments		241.8	226.5
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value		228.4	203.7
Bamboo CRV
Concentration Risk [Line Items]
Pledged collateral		24.7	46.7
Bamboo Captive
Concentration Risk [Line Items]
Pledged collateral		38.9	$ 32.2
Distinguished Transaction
Concentration Risk [Line Items]
Pledged collateral		$ 47.5
MediaAlpha
Concentration Risk [Line Items]
Shares purchased (in shares) | shares	7.6
Purchase price (in dollars per share) | $ / shares	$ 19.00	$ 11.38	$ 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)		27.40%	26.60%
Fully diluted ownership interest (as a percent)		25.40%
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		$ 106.7	$ 59.2