Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2023 $ / shares shares	Dec. 31, 2024 USD ($) fund $ / shares shares	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($)
Concentration Risk [Line Items]
Gross realized investment gains			$ 139.1	$ 113.7	$ 129.9
Loss on sale of investment securities			21.1	16.9	14.8
Proceeds from sale and maturity of investments			$ 1,200.0	$ 800.0	$ 500.0
Weighted average portfolio period			1 year 10 months 24 days	1 year 10 months 24 days
Weighted average portfolio period, excluding short-term invetments			2 years 7 months 6 days	3 years 3 months 18 days
Other long-term investments			$ 2,150.2	$ 1,998.2
Total investments			6,477.6	6,388.7
Level 3 | Recent transaction
Concentration Risk [Line Items]
Other long-term investments			23.4	19.0
Private equity funds
Concentration Risk [Line Items]
Other long-term investments			$ 249.5
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
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments			$ 264.7	194.4
Other long-term investments, redemption, written notice period			15 days
Foreign Regulatory Deposits
Concentration Risk [Line Items]
Other long-term investments			$ 149.9	158.0
ILS funds
Concentration Risk [Line Items]
Other long-term investments			$ 74.0	160.5
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			$ 236.7	209.1
Collateralized loan obligations | Level 3
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			0.0	0.0
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments			201.6	254.9
Fair value measured on a recurring basis | MediaAlpha | Level 3
Concentration Risk [Line Items]
Other long-term investments			0.0	0.0
Fair value measured on a recurring basis | Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			236.7	209.1
Fair value measured on a recurring basis | Collateralized loan obligations | Level 3
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			0.0	0.0
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments			149.9	158.0
Lloyds Bank
Concentration Risk [Line Items]
Total investments			361.5	345.2
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments			226.5	244.5
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value			203.7	$ 265.3
Bamboo CRV
Concentration Risk [Line Items]
Restricted cash			32.2
Pledged collateral			$ 46.7
MediaAlpha
Concentration Risk [Line Items]
Shares purchased (in shares) | shares	7.6	5.9
Purchase price (in dollars per share) | $ / shares	$ 19.00	$ 10.00	$ 11.29	$ 11.15
Net of underwriting fees (in dollars per share) | $ / shares	$ 18.24
Investment owned (in shares) | shares			17.9
Ownership interest (as a percent)			26.60%	34.90%
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]
Number of shares issued (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			$ 59.2	$ 57.3