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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2023 USD ($) $ / shares	Sep. 30, 2023 USD ($) fund $ / shares shares	Dec. 31, 2022 USD ($)
Concentration Risk [Line Items]
Percentage of investment fair value		68.00%	72.00%
Investments held in trusts		$ 531.8	$ 500.5
Assets held by insurance regulators		4.6	4.6
Other long-term investments		$ 1,895.8	1,488.0
MediaAlpha \| IPO
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) \| $ / shares		$ 9.00
Fair value measured on a recurring basis \| MediaAlpha
Concentration Risk [Line Items]
Other long-term investments		$ 188.8	168.6
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		150.3	137.4
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments		335.2	319.2
Outrigger Re
Concentration Risk [Line Items]
Short-term investments		$ 233.3	$ 203.7
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price	$ 5.9
Purchase price (in dollars per share) \| $ / shares	$ 10.00	$ 8.26
Investment owned (in shares) \| shares		22.9
Ownership interest (as a percent)		35.30%	27.10%
Fully diluted ownership interest (as a percent)		33.00%
Increase in investment owned (in dollars per share) \| $ / shares		$ 1.00
Largest Investment \| Private Equity and Hedge Fund Concentration Risk \| Investments
Concentration Risk [Line Items]
Other long-term investments		$ 53.4	$ 49.0
HG Global Senior Notes \| Senior Notes
Concentration Risk [Line Items]
Interest reserve account		29.9	31.2
Kudu Credit Facility \| Secured Debt
Concentration Risk [Line Items]
Interest reserve account		14.1
Restricted cash			12.2
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments		147.2	90.3
Asset Pledged as Collateral \| Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments		232.4	257.0
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		191.6	182.9
Collateralized loan obligations \| Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		191.6	182.9
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 239.2
Number of investments \| fund		18
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		$ 189.3	174.8
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments		150.3	137.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 159.5	$ 49.3
ILS funds \| Minimum
Concentration Risk [Line Items]
Notice period		30 days
ILS funds \| Maximum
Concentration Risk [Line Items]
Notice period		90 days