Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2026 USD ($) infrastructureFund fund $ / shares shares	Dec. 31, 2025 USD ($) $ / shares shares
Concentration Risk [Line Items]
Investment portfolio duration	2 years 9 months 18 days	2 years 6 months
Weighted average portfolio period, excluding short-term investment	2 years	1 year 6 months
Other long-term investments	$ 3,417.5	$ 2,958.5
Total investments	8,280.9	8,323.9
Private equity funds
Concentration Risk [Line Items]
Other long-term investments	$ 257.1
Number of investments \| fund	18
Unfunded Commitments	$ 81.1	65.8
Hedge funds
Concentration Risk [Line Items]
Number of investments \| fund	2
Unfunded Commitments	$ 0.0	0.0
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	$ 318.7	308.5
Written notice period	15 days
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	$ 185.0	180.4
ILS funds
Concentration Risk [Line Items]
Other long-term investments	$ 54.9	50.1
ILS funds \| Minimum
Concentration Risk [Line Items]
Notice period	30 days
ILS funds \| Maximum
Concentration Risk [Line Items]
Notice period	90 days
Equity Infrastructure Funds
Concentration Risk [Line Items]
Other long-term investments	$ 100.0
Written notice period	90 days
Number of infrastructure funds \| infrastructureFund	2
Prefunded commitments	$ 200.0
Equity Infrastructure Funds \| Minimum
Concentration Risk [Line Items]
Lock up period	0 years
Equity Infrastructure Funds \| Maximum
Concentration Risk [Line Items]
Lock up period	4 years
Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	$ 378.4	347.0
Fair value measured on a recurring basis \| MediaAlpha
Concentration Risk [Line Items]
Other long-term investments	170.9	231.2
Fair value measured on a recurring basis \| Collateralized loan obligations
Concentration Risk [Line Items]
Fixed maturity investments, at fair value	378.4	347.0
Lloyd’s trust deposits
Concentration Risk [Line Items]
Other long-term investments	185.0	180.4
Lloyds Bank
Concentration Risk [Line Items]
Total investments	511.8	361.6
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Concentration Risk [Line Items]
Total investments	360.5	257.2
Outrigger Re
Concentration Risk [Line Items]
Short-term investments, at fair value	48.0	245.7
Distinguished Transaction
Concentration Risk [Line Items]
Pledged collateral	$ 57.9	$ 56.8
MediaAlpha
Concentration Risk [Line Items]
Investment owned (in shares) \| shares	17.9	17.9
Ownership interest (as a percent)	29.30%	27.70%
Purchase price (in dollars per share) \| $ / shares	$ 9.57	$ 12.95
Largest Investment \| Investments \| Private Equity and Hedge Fund Concentration Risk
Concentration Risk [Line Items]
Other long-term investments	$ 166.8	$ 110.6

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Investment Redemption, Notice Period

+ References

No definition available.

+ Details

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- Definition

Number of Investments

+ References

No definition available.

+ Details

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- Definition

Number of Prefunded Infrastructure Funds

+ References

No definition available.

+ Details

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- Definition

Other Long-Term Investments, Redemption, Written Notice Period

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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- Details

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