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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Sep. 30, 2024 USD ($) security | Dec. 31, 2023 USD ($) security | Dec. 31, 2022 USD ($) security Security
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 244.7 | $ 196.5 | $ 2,321.4
Gross unrealized losses, less than twelve months | (1.2) | (2.1) | (145.4)
Over 12 months, Fair Market Value | 2,169.1 | 2,802.0 | 1,323.7
Over 12 months, Gross Unrealized Losses | (142.3) | (219.7) | (193.6)
Total, Fair Market Value | $ 2,413.8 | 2,998.5 | 3,645.1
Total, Gross Unrealized Losses |  | $ (221.8) | $ (339.0)
Number of Securities | security | 781 | 950 | 1,166
Fixed income securities — Available for sale |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value |  |  | $ 2,269.5
Gross unrealized losses, less than twelve months |  |  | (145.0)
Over 12 months, Fair Market Value |  |  | 1,323.7
Over 12 months, Gross Unrealized Losses |  |  | (193.6)
Total, Fair Market Value |  |  | 3,593.2
Total, Gross Unrealized Losses |  |  | $ (338.6)
Number of Securities | Security |  |  | 1,165
Short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value |  |  | $ 51.9
Gross unrealized losses, less than twelve months |  |  | (0.4)
Over 12 months, Fair Market Value |  |  | 0.0
Over 12 months, Gross Unrealized Losses |  |  | 0.0
Total, Fair Market Value |  |  | 51.9
Total, Gross Unrealized Losses |  |  | $ (0.4)
Number of Securities | Security |  |  | 1
U.S. government |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 126.0 | $ 105.5 | $ 741.1
Gross unrealized losses, less than twelve months | (0.7) | (0.5) | (30.6)
Over 12 months, Fair Market Value | 560.2 | 673.3 | 203.4
Over 12 months, Gross Unrealized Losses | (13.1) | (26.2) | (20.1)
Total, Fair Market Value | $ 686.2 | 778.8 | 944.5
Total, Gross Unrealized Losses |  | $ (26.7) | $ (50.7)
Number of Securities | security | 65 | 74 | 118
U.S. agency |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 0.0 | $ 0.0 | $ 7.1
Gross unrealized losses, less than twelve months | 0.0 | 0.0 | (0.4)
Over 12 months, Fair Market Value | 7.4 | 7.2 | 1.7
Over 12 months, Gross Unrealized Losses | (0.1) | (0.3) | 0.0
Total, Fair Market Value | $ 7.4 | 7.2 | 8.8
Total, Gross Unrealized Losses |  | $ (0.3) | $ (0.4)
Number of Securities | security | 1 | 1 | 5
Municipal |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 0.0 | $ 11.1 | $ 133.1
Gross unrealized losses, less than twelve months | 0.0 | (1.0) | (7.1)
Over 12 months, Fair Market Value | 66.2 | 117.0 | 16.4
Over 12 months, Gross Unrealized Losses | (1.6) | (4.0) | (2.3)
Total, Fair Market Value | $ 66.2 | 128.1 | 149.5
Total, Gross Unrealized Losses |  | $ (5.0) | $ (9.4)
Number of Securities | security | 37 | 54 | 59
Corporate |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 59.0 | $ 46.7 | $ 1,104.7
Gross unrealized losses, less than twelve months | (0.3) | (0.4) | (77.6)
Over 12 months, Fair Market Value | 920.7 | 1,287.2 | 568.2
Over 12 months, Gross Unrealized Losses | (58.2) | (101.1) | (91.5)
Total, Fair Market Value | $ 979.7 | 1,333.9 | 1,672.9
Total, Gross Unrealized Losses |  | $ (101.5) | $ (169.1)
Number of Securities | security | 435 | 558 | 701
Non-U.S. government-backed corporate |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 4.2 | $ 0.2 | $ 11.3
Gross unrealized losses, less than twelve months | 0.0 | 0.0 | (0.5)
Over 12 months, Fair Market Value | 102.9 | 95.5 | 99.0
Over 12 months, Gross Unrealized Losses | (3.1) | (5.9) | (8.3)
Total, Fair Market Value | $ 107.1 | 95.7 | 110.3
Total, Gross Unrealized Losses |  | $ (5.9) | $ (8.8)
Number of Securities | security | 12 | 12 | 16
Non-U.S government |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 23.6 | $ 32.9 | $ 63.8
Gross unrealized losses, less than twelve months | 0.0 | (0.2) | (2.9)
Over 12 months, Fair Market Value | 114.6 | 180.1 | 135.9
Over 12 months, Gross Unrealized Losses | (2.2) | (6.5) | (8.6)
Total, Fair Market Value | $ 138.2 | 213.0 | 199.7
Total, Gross Unrealized Losses |  | $ (6.7) | $ (11.5)
Number of Securities | security | 37 | 53 | 45
Agency commercial mortgage-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 0.0 | $ 0.0 | $ 5.6
Gross unrealized losses, less than twelve months | 0.0 | 0.0 | (1.0)
Over 12 months, Fair Market Value | 4.6 | 5.8 | 0.0
Over 12 months, Gross Unrealized Losses | (0.5) | (0.8) | 0.0
Total, Fair Market Value | $ 4.6 | 5.8 | 5.6
Total, Gross Unrealized Losses |  | $ (0.8) | $ (1.0)
Number of Securities | security | 1 | 1 | 1
Residential Mortgage-Backed Securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value | $ 13.1 | $ 0.1 |
Gross unrealized losses, less than twelve months | 0.0 | 0.0 |
Over 12 months, Fair Market Value | 392.5 | 435.9 |
Over 12 months, Gross Unrealized Losses | (63.5) | (74.9) |
Total, Fair Market Value | $ 405.6 | 436.0 |
Total, Gross Unrealized Losses |  | $ (74.9) |
Number of Securities | security | 181 | 197 |
Agency mortgage-backed |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
0-12 months, Fair Market Value |  |  | $ 202.8
Gross unrealized losses, less than twelve months |  |  | (24.9)
Over 12 months, Fair Market Value |  |  | 299.1
Over 12 months, Gross Unrealized Losses |  |  | (62.8)
Total, Fair Market Value |  |  | 501.9
Total, Gross Unrealized Losses |  |  | $ (87.7)
Number of Securities | security |  |  | 220

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