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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
	Mar. 28, 2024 USD ($) $ / shares	Mar. 31, 2024 USD ($) fund $ / shares shares	Jun. 30, 2023 USD ($) $ / shares	Dec. 31, 2023 USD ($)
Concentration Risk [Line Items]
Investments held in trusts		$ 589.6		$ 585.6
Assets held by insurance regulators		4.7		4.6
Other long-term investments		$ 1,991.9		1,998.2
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	$ 465.6	$ 465.6		254.9
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		150.5		158.0
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments		345.6		344.7
Outrigger Re
Concentration Risk [Line Items]
Short-term investments		222.4		$ 265.3
Bamboo Captive
Concentration Risk [Line Items]
Short-term investments		$ 20.7
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price			$ 5.9
Purchase price (in dollars per share) \| $ / shares	$ 20.37		$ 10.00
Investment owned (in shares) \| shares		22.9
Ownership interest (as a percent)		34.70%		34.90%
Fully diluted ownership interest (as a percent)		32.40%
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		$ 59.7		$ 57.3
HG Global Senior Notes \| Senior Notes
Concentration Risk [Line Items]
Interest reserve account		30.5		30.4
Kudu Credit Facility \| Secured Debt
Concentration Risk [Line Items]
Restricted cash		15.1		14.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments		201.9		198.9
Asset Pledged as Collateral \| Additional Investments, Insurance Regulators and Reinsurance Counterparties
Concentration Risk [Line Items]
Fair value investments		204.6		244.5
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		246.5		209.1
Collateralized loan obligations \| Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		246.5		209.1
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 255.2
Number of investments \| fund		16
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		$ 249.6		194.4
Foreign regulatory deposits
Concentration Risk [Line Items]
Other long-term investments		150.5		158.0
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 109.7		$ 160.5
ILS funds \| Minimum
Concentration Risk [Line Items]
Notice period		30 days
ILS funds \| Maximum
Concentration Risk [Line Items]
Notice period		90 days