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, 2025 USD ($) fund $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2023 USD ($)
Concentration Risk [Line Items]
Gross realized investment gains			$ 117.1	$ 139.1	$ 113.7
Loss on sale of investment securities			5.6	21.1	16.9
Proceeds from sale and maturity of investments			$ 1,400.0	$ 1,200.0	$ 800.0
Weighted average portfolio period			1 year 6 months	1 year 10 months 24 days
Weighted average portfolio period, excluding short-term invetments			2 years 6 months	2 years 7 months 6 days
Other long-term investments			$ 2,958.5	$ 2,150.2
Total investments			8,323.9	6,417.6
Level 3 | Recent transaction
Concentration Risk [Line Items]
Other long-term investments			250.4	23.4
Private equity funds
Concentration Risk [Line Items]
Other long-term investments			$ 251.2
Number of investments | fund			17
Unfunded Commitments			$ 65.8	83.7
Hedge funds
Concentration Risk [Line Items]
Number of investments | fund			2
Unfunded Commitments			$ 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			$ 308.5	264.7
Other long-term investments, redemption, written notice period			15 days
Foreign Regulatory Deposits
Concentration Risk [Line Items]
Other long-term investments			$ 180.4	149.9
ILS funds
Concentration Risk [Line Items]
Other long-term investments			$ 50.1	74.0
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period			30 days
Infrastructure Fund
Concentration Risk [Line Items]
Other long-term investments, redemption, written notice period			90 days
Unfunded Commitments			$ 100.0
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			$ 347.0	236.7
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			231.2	201.6
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			347.0	236.7
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			180.4	149.9
Lloyds Bank
Concentration Risk [Line Items]
Total investments			361.6	361.5
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments			257.2	226.5
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value			245.7	203.7
Bamboo Captive
Concentration Risk [Line Items]
Pledged collateral				32.2
Bamboo CRV
Concentration Risk [Line Items]
Pledged collateral			25.7	$ 46.7
Distinguished Transaction
Concentration Risk [Line Items]
Pledged collateral			$ 56.8
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	$ 12.95	$ 11.29
Net of underwriting fees (in dollars per share) | $ / shares	$ 18.24
Investment owned (in shares) | shares			17.9	17.9
Ownership interest (as a percent)			27.40%	26.60%
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			$ 110.6	$ 59.2