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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($) fund $ / shares shares	Jun. 30, 2024 USD ($) fund $ / shares shares	Jun. 30, 2023 USD ($) $ / shares	Jun. 30, 2024 USD ($) fund $ / shares shares	Dec. 31, 2023 USD ($)
Concentration Risk [Line Items]
Other long-term investments	$ 2,022.4	$ 2,022.4		$ 2,022.4	$ 1,998.2
Investments held in trusts	611.8	611.8		611.8	585.6
Assets held by insurance regulators	4.8	4.8		$ 4.8	4.6
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	235.2	235.2		$ 235.2	254.9
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	156.1	156.1		156.1	158.0
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments	353.4	353.4		353.4	344.7
Outrigger Re
Concentration Risk [Line Items]
Short-term investments	238.6	238.6		238.6	$ 265.3
Bamboo Captive
Concentration Risk [Line Items]
Short-term investments	28.9	28.9		28.9
Bamboo CRV
Concentration Risk [Line Items]
Pledged collateral	$ 12.0	12.0		$ 12.0
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price		$ 7.6	$ 5.9
Purchase price (in dollars per share) | $ / shares	$ 13.17	$ 19.00	$ 10.00
Purchase price net of underwriting fees (in dollars per share) | $ / shares		$ 18.24
Investment owned (in shares) | shares	17.9	17.9		17.9
Ownership interest (as a percent)	26.90%	26.90%		26.90%	34.90%
Fully diluted ownership interest (as a percent)	25.40%	25.40%		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	$ 60.1	60.1		$ 60.1	$ 57.3
HG Global Senior Notes | Senior Notes
Concentration Risk [Line Items]
Interest reserve account	29.4	29.4		29.4	30.4
Kudu Credit Facility | Secured Debt
Concentration Risk [Line Items]
Restricted cash	15.0	15.0		15.0	14.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments	203.0	203.0		203.0	198.9
Asset Pledged as Collateral | Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments	206.0	206.0		206.0	244.5
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	270.0	270.0		270.0	209.1
Collateralized loan obligations | Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	270.0	270.0		270.0	209.1
Private equity funds
Concentration Risk [Line Items]
Other long-term investments	$ 264.0	$ 264.0		$ 264.0
Number of investments | fund	17	17		17
Hedge funds
Concentration Risk [Line Items]
Number of investments | fund	1	1		1
Notice period				45 days
Bank loan fund
Concentration Risk [Line Items]
Other long-term investments	$ 254.3	$ 254.3		$ 254.3	194.4
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments	156.1	156.1		156.1	158.0
ILS funds
Concentration Risk [Line Items]
Other long-term investments	$ 110.2	$ 110.2		$ 110.2	$ 160.5
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period				30 days
ILS funds | Maximum
Concentration Risk [Line Items]
Notice period				90 days

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