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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2022 USD ($) security Security | Dec. 31, 2021 USD ($) security Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 2,321.4 | $ 2,014.6
Gross unrealized losses, less than twelve months | (145.4) | (30.9)
Over 12 months, Fair Market Value | 1,323.7 | 72.0
Over 12 months, Gross Unrealized Losses | (193.6) | (3.6)
Total, Fair Market Value | 3,645.1 | 2,086.6
Total, Gross Unrealized Losses | $ (339.0) | $ (34.5)
Number of Securities | security | 1,166 | 644
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 2,269.5 | $ 2,010.2
Gross unrealized losses, less than twelve months | (145.0) | (30.9)
Over 12 months, Fair Market Value | 1,323.7 | 72.0
Over 12 months, Gross Unrealized Losses | (193.6) | (3.6)
Total, Fair Market Value | 3,593.2 | 2,082.2
Total, Gross Unrealized Losses | $ (338.6) | $ (34.5)
Number of Securities | Security | 1,165 | 637
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 51.9 | $ 4.4
Gross unrealized losses, less than twelve months | (0.4) | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 51.9 | 4.4
Total, Gross Unrealized Losses | $ (0.4) | $ 0.0
Number of Securities | 1 | 7
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 741.1 | $ 309.1
Gross unrealized losses, less than twelve months | (30.6) | (4.0)
Over 12 months, Fair Market Value | 203.4 | 29.3
Over 12 months, Gross Unrealized Losses | (20.1) | (1.9)
Total, Fair Market Value | 944.5 | 338.4
Total, Gross Unrealized Losses | $ (50.7) | $ (5.9)
Number of Securities | security | 118 | 51
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 7.1 | $ 6.1
Gross unrealized losses, less than twelve months | (0.4) | 0.0
Over 12 months, Fair Market Value | 1.7 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 8.8 | 6.1
Total, Gross Unrealized Losses | $ (0.4) | $ 0.0
Number of Securities | security | 5 | 5
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 133.1 | $ 29.7
Gross unrealized losses, less than twelve months | (7.1) | (0.5)
Over 12 months, Fair Market Value | 16.4 | 0.0
Over 12 months, Gross Unrealized Losses | (2.3) | 0.0
Total, Fair Market Value | 149.5 | 29.7
Total, Gross Unrealized Losses | $ (9.4) | $ (0.5)
Number of Securities | security | 59 | 20
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,104.7 | $ 804.6
Gross unrealized losses, less than twelve months | (77.6) | (11.7)
Over 12 months, Fair Market Value | 568.2 | 23.6
Over 12 months, Gross Unrealized Losses | (91.5) | (1.3)
Total, Fair Market Value | 1,672.9 | 828.2
Total, Gross Unrealized Losses | $ (169.1) | $ (13.0)
Number of Securities | security | 701 | 400
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 11.3 | $ 114.4
Gross unrealized losses, less than twelve months | (0.5) | (1.9)
Over 12 months, Fair Market Value | 99.0 | 0.0
Over 12 months, Gross Unrealized Losses | (8.3) | 0.0
Total, Fair Market Value | 110.3 | 114.4
Total, Gross Unrealized Losses | $ (8.8) | $ (1.9)
Number of Securities | security | 16 | 10
Non-U.S government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 63.8 | $ 181.8
Gross unrealized losses, less than twelve months | (2.9) | (1.2)
Over 12 months, Fair Market Value | 135.9 | 3.1
Over 12 months, Gross Unrealized Losses | (8.6) | 0.0
Total, Fair Market Value | 199.7 | 184.9
Total, Gross Unrealized Losses | $ (11.5) | $ (1.2)
Number of Securities | security | 45 | 35
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 202.8 | $ 564.5
Gross unrealized losses, less than twelve months | (24.9) | (11.6)
Over 12 months, Fair Market Value | 299.1 | 16.0
Over 12 months, Gross Unrealized Losses | (62.8) | (0.4)
Total, Fair Market Value | 501.9 | 580.5
Total, Gross Unrealized Losses | $ (87.7) | $ (12.0)
Number of Securities | security | 220 | 116

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