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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | --- | ---
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | $ 4,657.4 | $ 4,330.0 | $ 4,131.3
Gross unrealized gains | 43.9 | 17.3 | 3.6
Gross unrealized losses | (143.5) | (221.8) | (338.6)
Allowance for credit losses | (2.0) | (2.9) | (7.7)
Fair market value | 4,555.8 | 4,122.6 | 3,788.6
Total short-term investments — Available for sale |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Gross unrealized gains | 0.0 | 0.0 | 0.0
Gross unrealized losses | 0.0 | 0.0 | (0.4)
Allowance for credit losses | 0.0 | 0.0 | 0.0
Fixed maturities |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 4,749.0 | 4,438.3 | 4,183.7
Gross unrealized gains | 44.1 | 17.5 | 3.6
Gross unrealized losses | (143.5) | (221.8) | (339.0)
Allowance for credit losses | (2.0) | (2.9) | (7.7)
Fair market value | 4,647.6 | 4,231.1 | 3,840.6
U.S. government |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 1,329.2 | 1,224.9 | 1,003.7
Gross unrealized gains | 11.0 | 4.4 | 0.0
Gross unrealized losses | (13.8) | (26.7) | (50.7)
Allowance for credit losses | 0.0 | 0.0 | 0.0
Fair market value | 1,326.4 | 1,202.6 | 953.0
U.S. agency |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 7.5 | 7.5 | 9.2
Gross unrealized gains | 0.0 | 0.0 | 0.0
Gross unrealized losses | (0.1) | (0.3) | (0.4)
Allowance for credit losses | 0.0 | 0.0 | 0.0
Fair market value | 7.4 | 7.2 | 8.8
Municipal |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 81.2 | 133.6 | 159.9
Gross unrealized gains | 0.1 | 0.0 | 0.0
Gross unrealized losses | (1.6) | (5.0) | (9.4)
Allowance for credit losses | (0.3) | (0.5) | (1.0)
Fair market value | 79.4 | 128.1 | 149.5
Corporate |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 1,999.9 | 2,051.1 | 2,016.9
Gross unrealized gains | 24.5 | 12.1 | 3.5
Gross unrealized losses | (58.5) | (101.5) | (169.1)
Allowance for credit losses | (1.7) | (2.4) | (6.3)
Fair market value | 1,964.2 | 1,959.3 | 1,845.0
Non-U.S. government-backed corporate |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 146.1 | 106.5 | 119.4
Gross unrealized gains | 1.2 | 0.1 | 0.0
Gross unrealized losses | (3.1) | (5.9) | (8.8)
Allowance for credit losses | 0.0 | 0.0 | (0.2)
Fair market value | 144.2 | 100.7 | 110.4
Non-U.S government |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 273.0 | 279.9 | 225.2
Gross unrealized gains | 2.1 | 0.6 | 0.1
Gross unrealized losses | (2.2) | (6.7) | (11.5)
Allowance for credit losses | 0.0 | 0.0 | (0.2)
Fair market value | 272.9 | 273.8 | 213.6
Agency commercial mortgage-backed securities |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 5.0 | 6.6 | 6.6
Gross unrealized gains | 0.0 | 0.0 | 0.0
Gross unrealized losses | (0.5) | (0.8) | (1.0)
Allowance for credit losses | 0.0 | 0.0 | 0.0
Fair market value | 4.5 | 5.8 | 5.6
Agency mortgage-backed |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost |  | 519.9 | 590.4
Gross unrealized gains |  |  | 0.0
Gross unrealized losses |  |  | (87.7)
Allowance for credit losses |  |  | 0.0
Fair market value |  | 445.1 | 502.7
Residential Mortgage-Backed Securities |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost | 616.6 | 519.9 |
Gross unrealized gains | 3.6 | 0.1 |
Gross unrealized losses | (63.5) | (74.9) |
Allowance for credit losses | 0.0 | 0.0 |
Fair market value | 556.7 | 445.1 |
Total short-term investments — Available for sale |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost |  | 93.6 | 52.4
Fair market value |  | 93.6 | $ 52.0
Privately-held investments, available for sale |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |
Fixed income securities, available for sale, amortized cost |  | 14.7 |
Gross unrealized gains | 0.2 | 0.2 |
Gross unrealized losses | 0.0 | 0.0 |
Allowance for credit losses | 0.0 | 0.0 |
Fair market value | $ 25.3 | $ 14.9 |

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