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v3.23.2

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Jun. 30, 2023 USD ($) security Security | Dec. 31, 2022 USD ($) security Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,175.6 | $ 2,321.4
Gross unrealized losses, less than twelve months | (32.5) | (145.4)
Over 12 months, Fair Market Value | 2,582.7 | 1,323.7
Over 12 months, Gross Unrealized Losses | (288.9) | (193.6)
Total, Fair Market Value | 3,758.3 | 3,645.1
Total, Gross Unrealized Losses | $ (321.4) | $ (339.0)
Number of Securities | security | 1,193 | 1,166
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,163.8 | $ 2,269.5
Gross unrealized losses, less than twelve months | (32.5) | (145.0)
Over 12 months, Fair Market Value | 2,582.7 | 1,323.7
Over 12 months, Gross Unrealized Losses | (288.9) | (193.6)
Total, Fair Market Value | 3,746.5 | 3,593.2
Total, Gross Unrealized Losses | $ (321.4) | $ (338.6)
Number of Securities | Security | 1,188 | 1,165
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 11.8 | $ 51.9
Gross unrealized losses, less than twelve months | 0.0 | (0.4)
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 11.8 | 51.9
Total, Gross Unrealized Losses | $ 0.0 | $ (0.4)
Number of Securities | 5 | 1
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 494.8 | $ 741.1
Gross unrealized losses, less than twelve months | (12.8) | (30.6)
Over 12 months, Fair Market Value | 519.1 | 203.4
Over 12 months, Gross Unrealized Losses | (38.3) | (20.1)
Total, Fair Market Value | 1,013.9 | 944.5
Total, Gross Unrealized Losses | $ (51.1) | $ (50.7)
Number of Securities | security | 105 | 118
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 7.0 | $ 7.1
Gross unrealized losses, less than twelve months | (0.4) | (0.4)
Over 12 months, Fair Market Value | 1.1 | 1.7
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 8.1 | 8.8
Total, Gross Unrealized Losses | $ (0.4) | $ (0.4)
Number of Securities | security | 4 | 5
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 90.9 | $ 133.1
Gross unrealized losses, less than twelve months | (3.7) | (7.1)
Over 12 months, Fair Market Value | 58.7 | 16.4
Over 12 months, Gross Unrealized Losses | (5.1) | (2.3)
Total, Fair Market Value | 149.6 | 149.5
Total, Gross Unrealized Losses | $ (8.8) | $ (9.4)
Number of Securities | security | 60 | 59
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 467.4 | $ 1,104.7
Gross unrealized losses, less than twelve months | (13.1) | (77.6)
Over 12 months, Fair Market Value | 1,254.7 | 568.2
Over 12 months, Gross Unrealized Losses | (142.1) | (91.5)
Total, Fair Market Value | 1,722.1 | 1,672.9
Total, Gross Unrealized Losses | $ (155.2) | $ (169.1)
Number of Securities | security | 727 | 701
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 5.2 | $ 11.3
Gross unrealized losses, less than twelve months | 0.0 | (0.5)
Over 12 months, Fair Market Value | 105.7 | 99.0
Over 12 months, Gross Unrealized Losses | (8.7) | (8.3)
Total, Fair Market Value | 110.9 | 110.3
Total, Gross Unrealized Losses | $ (8.7) | $ (8.8)
Number of Securities | security | 16 | 16
Non-U.S government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 80.1 | $ 63.8
Gross unrealized losses, less than twelve months | (1.2) | (2.9)
Over 12 months, Fair Market Value | 182.2 | 135.9
Over 12 months, Gross Unrealized Losses | (10.8) | (8.6)
Total, Fair Market Value | 262.3 | 199.7
Total, Gross Unrealized Losses | $ (12.0) | $ (11.5)
Number of Securities | security | 61 | 45
Non-agency commercial mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 0.0 | $ 5.6
Gross unrealized losses, less than twelve months | 0.0 | (1.0)
Over 12 months, Fair Market Value | 5.6 | 0.0
Over 12 months, Gross Unrealized Losses | (0.9) | 0.0
Total, Fair Market Value | 5.6 | 5.6
Total, Gross Unrealized Losses | $ (0.9) | $ (1.0)
Number of Securities | security | 1 | 1
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 18.4 | $ 202.8
Gross unrealized losses, less than twelve months | (1.3) | (24.9)
Over 12 months, Fair Market Value | 455.6 | 299.1
Over 12 months, Gross Unrealized Losses | (83.0) | (62.8)
Total, Fair Market Value | 474.0 | 501.9
Total, Gross Unrealized Losses | $ (84.3) | $ (87.7)
Number of Securities | security | 214 | 220

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