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Investments (Tables)
 9 Months Ended
Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract]  
Schedule of Debt Securities, Available-for-Sale
The following table summarizes the fair value of fixed maturity investments:
September 30, 2025
Amortized CostUnrealized
gains		Unrealized
losses		Fair
value
U.S. Treasuries$436.8 $6.4 $(0.2)$443.0 
Agencies6.8 — — 6.8 
Non-U.S. government40.6 0.6 — 41.2 
Corporate bonds1,682.2 38.2 (0.2)1,720.2 
Residential mortgage-backed245.2 5.4 — 250.6 
Commercial mortgage-backed0.6 — — 0.6 
Other asset-backed securities246.0 2.2 (0.1)248.1 
Total fixed maturity securities$2,658.2 $52.8 $(0.5)$2,710.5 
December 31, 2024
Amortized CostUnrealized
gains		Unrealized
losses		Fair
value
U.S. Treasuries$747.6 $2.1 $(3.1)$746.6 
Agencies11.5 — — 11.5 
Non-U.S. government46.6 0.1 (0.3)46.4 
Corporate bonds1,906.3 10.9 (4.0)1,913.2 
Residential mortgage-backed279.5 0.8 (1.2)279.1 
Commercial mortgage-backed— 0.4 — 0.4 
Other asset-backed securities412.3 2.3 (0.2)414.4 
Total fixed maturity securities$3,403.8 $16.6 $(8.8)$3,411.6 
The composition of the fair values of fixed maturity securities by credit rating is as follows:
September 30, 2025December 31, 2024
Fair Value%Fair Value%
AAA$239.5 9%$399.4 12%
AA866.1 32%1,245.6 37%
A1,144.8 42%1,270.9 37%
BBB432.7 16%453.1 13%
Below BBB27.4 1%42.6 1%
Total fixed maturity securities$2,710.5 100%$3,411.6 100%
The contractual maturities for fixed maturity securities are listed in the following table:
September 30, 2025December 31, 2024
Amortized CostFair ValueAmortized CostFair Value
Due in one year or less$226.1 $227.6 $147.9 $147.6 
Due after one year through five years1,554.4 1,584.3 2,141.2 2,149.8 
Due after five years through ten years545.2 560.2 767.7 766.7 
Due after ten years332.5 338.4 347.0 347.5 
Total fixed maturity securities$2,658.2 $2,710.5 $3,403.8 $3,411.6 
The Group’s short-term investments consist of U.S. Treasuries, corporate bonds and other asset-backed securities with maturities of 90 days or greater but less than one year at the time of purchase:
September 30, 2025
Amortized CostUnrealized
gains		Unrealized
losses		Fair
value
U.S. Treasuries$187.6 $0.1 $— $187.7 
Corporate bonds0.2 — — 0.2 
Total short-term investments
$187.8 $0.1 $— $187.9 
December 31, 2024
Amortized CostUnrealized
gains		Unrealized
losses		Fair
value
U.S. Treasuries$220.5 $0.2 $— $220.7 
Corporate bonds1.1 — — 1.1 
Other asset-backed securities0.3 — — 0.3 
Total short-term investments
$221.9 $0.2 $— $222.1 
The composition of the fair values of short-term investments by credit rating is as follows:
September 30, 2025December 31, 2024
Fair Value%Fair Value%
AAA$— —%$0.3 —%
AA187.7 100%220.7 100%
A0.2 —%— —%
BBB— —%0.2 —%
Below BBB— —%0.9 —%
Total short-term investments$187.9 100%$222.1 100%
Schedule of Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value
The following table summarizes, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Group’s available-for-sale portfolio:
September 30, 2025
0 - 12 months> 12 months
Fair
value		Gross unrealized lossesGross unrealized lossesNumber of securities
U.S. Treasuries$64.2 $(0.1)$(0.1)28 
Non-U.S. government1.2 — — 1 
Corporate bonds25.1 (0.1)(0.1)50 
Residential mortgage-backed10.0 — — 3 
Commercial mortgage-backed0.4 — — 1 
Other asset-backed securities26.7 (0.1)— 26 
Total$127.6 $(0.3)$(0.2)109 
December 31, 2024
0 - 12 months> 12 months
Fair
value		Gross unrealized lossesGross unrealized lossesNumber of securities
U.S. Treasuries$307.6 $(2.8)$(0.3)48 
Agencies4.0 — — 2 
Non-U.S. government23.7 (0.1)(0.2)11 
Corporate bonds543.6 (3.4)(0.6)310 
Residential mortgage-backed91.9 (1.1)(0.1)35 
Other asset-backed securities64.7 (0.2)— 25 
Total$1,035.5 $(7.6)$(1.2)431 
Schedule of Debt Securities, Available-for-Sale, Allowance for Credit Loss
The following table provides a roll forward of the allowance for expected credit losses of the Group’s securities classified as available-for-sale:
Three months endedNine months ended
September 30, 2025September 30, 2024September 30, 2025September 30, 2024
Balance at beginning of period$0.6 $3.2 $5.9 $1.3 
Expected credit losses on securities where credit losses were not previously recognized0.4 — 1.1 1.7 
Reductions for expected credit losses on securities where credit losses were previously recognized(0.6)(2.7)(5.8)(2.2)
Securities sold/redeemed/matured(0.1)(0.2)(0.9)(0.5)
Balance at end of period$0.3 $0.3 $0.3 $0.3 
Schedule of Other Investments Not Readily Marketable
The following table provides a summary of the Group’s other investments by investment strategy:
September 30, 2025December 31, 2024
Fair Value%Fair Value%
Hedge funds
Credit$24.5 6%$22.3 12%
Global macro42.5 11%44.8 22%
Long/short67.5 17%44.9 22%
Multi-strategy and event-driven95.7 24%89.0 44%
Multi-asset credit funds167.2 42%— —%
Total other investments
$397.4 100%$201.0 100%
Schedule of Investment Income
The components of net investment return are as follows:
Three Months EndedNine Months Ended
September 30, 2025September 30, 2024September 30, 2025September 30, 2024
Net interest and dividend income$47.9 $53.4 $146.2 $142.9 
Investment expenses(2.0)(1.3)(6.2)(3.8)
Net investment income45.9 52.1 140.0 139.1 
Net realized gains/(losses) on fixed maturity securities, available-for-sale1.3 (6.0)2.9 (19.5)
Net realized gains on other investments— 0.1 0.1 0.1 
Change in net unrealized gains on other investments4.7 2.5 10.3 1.9 
Change in net unrealized losses on derivatives(0.1)— (0.1)— 
Change in provision for expected credit losses 0.3 2.9 5.6 1.0 
Net realized and unrealized investment gains/(losses)6.2 (0.5)18.8 (16.5)
Total realized and unrealized investments gains/(losses) and net investment income$52.1 $51.6 $158.8 $122.6