Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2023 $ / shares shares	Sep. 30, 2024 USD ($) fund $ / shares shares	Dec. 31, 2023 USD ($)
Concentration Risk [Line Items]
Other long-term investments			$ 2,071.1	$ 1,998.2
Investments held in trusts			$ 641.4	585.6
MediaAlpha | IPO
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) | $ / shares			$ 7.00
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments			$ 323.4	254.9
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments			172.7	158.0
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments			356.8	344.7
Outrigger Re
Concentration Risk [Line Items]
Short-term investments			199.9	$ 265.3
Bamboo Captive
Concentration Risk [Line Items]
Short-term investments			32.9
Bamboo CRV
Concentration Risk [Line Items]
Pledged collateral			$ 12.1
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	$ 18.11
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)			26.80%	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]
Sale of stock, number of shares issued in transaction (in shares) | shares	5.0
Proceeds from sale of business	$ 91.2
Largest Investment | Private Equity and Hedge Fund Concentration Risk | Investments
Concentration Risk [Line Items]
Other long-term investments			$ 56.6	$ 57.3
HG Global Senior Notes | Senior Notes
Concentration Risk [Line Items]
Interest reserve account			29.3	30.4
Kudu Credit Facility | Secured Debt
Concentration Risk [Line Items]
Restricted cash			14.8	14.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments			208.0	198.9
Asset Pledged as Collateral | Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments			215.8	244.5
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			289.8	209.1
Collateralized loan obligations | Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value			289.8	209.1
Private equity funds
Concentration Risk [Line Items]
Other long-term investments			$ 262.0
Number of investments | fund			17
Hedge funds
Concentration Risk [Line Items]
Number of investments | fund			2
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments			$ 258.9	194.4
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments			172.7	158.0
ILS funds
Concentration Risk [Line Items]
Other long-term investments			$ 71.2	$ 160.5
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period			30 days
ILS funds | Maximum
Concentration Risk [Line Items]
Notice period			90 days