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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2024 USD ($) security | Dec. 31, 2023 USD ($) Security security
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Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,625.0 | $ 196.5
Gross unrealized losses, less than twelve months | (22.3) | (2.1)
Over 12 months, Fair Market Value | 1,604.9 | 2,802.0
Over 12 months, Gross Unrealized Losses | (159.6) | (219.7)
Total, Fair Market Value | 3,229.9 | 2,998.5
Total, Gross Unrealized Losses | $ (181.9) | $ (221.8)
Number of Securities | 1,170 | 950
Securities Below Investment Grade or NR | $ 1.2 |
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 0.1
Gross unrealized losses, less than twelve months |  | 0.0
Over 12 months, Fair Market Value |  | 435.9
Over 12 months, Gross Unrealized Losses |  | (74.9)
Total, Fair Market Value |  | 436.0
Total, Gross Unrealized Losses |  | $ (74.9)
Number of Securities | Security |  | 197
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | 718.1 | $ 105.5
Gross unrealized losses, less than twelve months | (11.6) | (0.5)
Over 12 months, Fair Market Value | 397.8 | 673.3
Over 12 months, Gross Unrealized Losses | (14.1) | (26.2)
Total, Fair Market Value | 1,115.9 | 778.8
Total, Gross Unrealized Losses | $ (25.7) | $ (26.7)
Number of Securities | security | 137 | 74
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 0.0 |
Gross unrealized losses, less than twelve months | 0.0 |
Over 12 months, Fair Market Value | 7.2 |
Over 12 months, Gross Unrealized Losses | (0.3) |
Total, Fair Market Value | 7.2 |
Total, Gross Unrealized Losses | $ (0.3) |
Number of Securities | security | 1 |
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 33.6 | $ 0.0
Gross unrealized losses, less than twelve months | (0.7) | 0.0
Over 12 months, Fair Market Value | 45.4 | 7.2
Over 12 months, Gross Unrealized Losses | (1.2) | (0.3)
Total, Fair Market Value | 79.0 | 7.2
Total, Gross Unrealized Losses | $ (1.9) | $ (0.3)
Number of Securities | security | 47 | 1
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 570.3 | $ 11.1
Gross unrealized losses, less than twelve months | (6.8) | (1.0)
Over 12 months, Fair Market Value | 663.9 | 117.0
Over 12 months, Gross Unrealized Losses | (59.6) | (4.0)
Total, Fair Market Value | 1,234.2 | 128.1
Total, Gross Unrealized Losses | $ (66.4) | $ (5.0)
Number of Securities | security | 692 | 54
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 5.8 | $ 46.7
Gross unrealized losses, less than twelve months | 0.0 | (0.4)
Over 12 months, Fair Market Value | 91.1 | 1,287.2
Over 12 months, Gross Unrealized Losses | (2.4) | (101.1)
Total, Fair Market Value | 96.9 | 1,333.9
Total, Gross Unrealized Losses | $ (2.4) | $ (101.5)
Number of Securities | security | 12 | 558
Non-U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 118.8 | $ 0.2
Gross unrealized losses, less than twelve months | (0.9) | 0.0
Over 12 months, Fair Market Value | 43.7 | 95.5
Over 12 months, Gross Unrealized Losses | (1.5) | (5.9)
Total, Fair Market Value | 162.5 | 95.7
Total, Gross Unrealized Losses | $ (2.4) | $ (5.9)
Number of Securities | security | 41 | 12
Non-agency commercial mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 0.0 |
Gross unrealized losses, less than twelve months | 0.0 |
Over 12 months, Fair Market Value | 4.5 |
Over 12 months, Gross Unrealized Losses | (0.6) |
Total, Fair Market Value | 4.5 |
Total, Gross Unrealized Losses | $ (0.6) |
Number of Securities | security | 1 |
Agency residential mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 161.2 |
Gross unrealized losses, less than twelve months | (2.2) |
Over 12 months, Fair Market Value | 351.3 |
Over 12 months, Gross Unrealized Losses | (79.9) |
Total, Fair Market Value | 512.5 |
Total, Gross Unrealized Losses | $ (82.1) |
Number of Securities | security | 225 |
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 0.0
Gross unrealized losses, less than twelve months |  | 0.0
Over 12 months, Fair Market Value |  | 5.8
Over 12 months, Gross Unrealized Losses |  | (0.8)
Total, Fair Market Value |  | 5.8
Total, Gross Unrealized Losses |  | $ (0.8)
Number of Securities | security |  | 1
Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 17.2 | $ 32.9
Gross unrealized losses, less than twelve months | (0.1) | (0.2)
Over 12 months, Fair Market Value | 0.0 | 180.1
Over 12 months, Gross Unrealized Losses | 0.0 | (6.5)
Total, Fair Market Value | 17.2 | 213.0
Total, Gross Unrealized Losses | $ (0.1) | $ (6.7)
Number of Securities | security | 14 | 53

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