Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2023 USD ($) $ / shares	Dec. 31, 2023 USD ($) fund $ / shares shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Concentration Risk [Line Items]
Gross realized investment gains		$ 113.7	$ 129.9	$ 212.3
Gross realized investment losses		(16.9)	(14.8)	(55.5)
Proceeds from sale and maturity of investments		$ 800.0	500.0	$ 800.0
Weighted average portfolio period		1 year 10 months 24 days
Weighted average portfolio period, excluding short-term invetments		3 years 3 months 18 days
Investments held in trusts		$ 585.6	500.5
Assets held by insurance regulators		4.6	4.6
Other long-term investments		1,998.2	1,488.0
Fair value investments		6,388.7	5,170.0
Private equity funds
Concentration Risk [Line Items]
Other long-term investments		$ 248.6
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		$ 194.4	174.8
Other long-term investments, redemption, written notice period		15 days
Foreign Regulatory Deposits
Concentration Risk [Line Items]
Other long-term investments		$ 158.0	137.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments		$ 160.5	49.3
ILS funds | Minimum
Concentration Risk [Line Items]
Notice period		30 days
MediaAlpha | IPO
Concentration Risk [Line Items]
Increase in book value per share (in dollars per share) | $ / shares		$ 9.00
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		$ 209.1	182.9
Fair value measured on a recurring basis
Concentration Risk [Line Items]
Fair value investments		6,388.7	5,170.0
Fair value measured on a recurring basis | MediaAlpha
Concentration Risk [Line Items]
Other long-term investments		254.9	168.6
Fair value measured on a recurring basis | Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value		209.1	182.9
Asset Pledged as Collateral
Concentration Risk [Line Items]
Short-term investments		198.9	90.3
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments		158.0	137.4
Lloyds Bank
Concentration Risk [Line Items]
Fair value investments		344.7	319.2
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral
Concentration Risk [Line Items]
Fair value investments		244.5	257.0
Outrigger Re
Concentration Risk [Line Items]
Short-term investments		$ 265.3	$ 203.7
MediaAlpha
Concentration Risk [Line Items]
Tender offer purchase price	$ 5.9
Purchase price (in dollars per share) | $ / shares	$ 10.00	$ 11.15
Investment owned (in shares) | shares		22.9
Ownership interest (as a percent)		34.90%	27.10%
Fully diluted ownership interest (as a percent)		33.10%
Increase in investment owned (in dollars per share) | $ / shares		$ 1.00
Largest Investment | Investments | Private Equity and Hedge Fund Concentration Risk
Concentration Risk [Line Items]
Other long-term investments		$ 57.3	$ 49.0
HG Global Senior Notes | Senior Notes
Concentration Risk [Line Items]
Interest reserve account		30.4	31.2
Kudu Credit Facility | Secured Debt
Concentration Risk [Line Items]
Interest reserve account		$ 14.9
Restricted cash			$ 12.2