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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2021 USD ($) security Security | Dec. 31, 2020 USD ($) security Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 2,014.6 | $ 147.8
Gross unrealized losses, less than twelve months | (30.9) | (0.8)
Over 12 months, Fair Market Value | 72.0 | 2.1
Over 12 months, Gross Unrealized Losses | (3.6) | (0.2)
Total, Fair Market Value | 2,086.6 | 149.9
Total, Gross Unrealized Losses | $ (34.5) | $ (1.0)
Number of Securities | security | 644 | 64
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 2,010.2 | $ 139.1
Gross unrealized losses, less than twelve months | (30.9) | (0.8)
Over 12 months, Fair Market Value | 72.0 | 2.1
Over 12 months, Gross Unrealized Losses | (3.6) | 0.0
Total, Fair Market Value | 2,082.2 | 141.2
Total, Gross Unrealized Losses | $ (34.5) | $ (0.8)
Number of Securities | Security | 637 | 49
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 4.4 | $ 8.7
Gross unrealized losses, less than twelve months | 0.0 | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | (0.2)
Total, Fair Market Value | 4.4 | 8.7
Total, Gross Unrealized Losses | $ 0.0 | $ (0.2)
Number of Securities | 7 | 15
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 309.1 | $ 58.7
Gross unrealized losses, less than twelve months | (4.0) | (0.6)
Over 12 months, Fair Market Value | 29.3 | 0.0
Over 12 months, Gross Unrealized Losses | (1.9) | 0.0
Total, Fair Market Value | 338.4 | 58.7
Total, Gross Unrealized Losses | $ (5.9) | $ (0.6)
Number of Securities | security | 51 | 10
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 6.1 |
Gross unrealized losses, less than twelve months | 0.0 |
Over 12 months, Fair Market Value | 0.0 |
Over 12 months, Gross Unrealized Losses | 0.0 |
Total, Fair Market Value | 6.1 |
Total, Gross Unrealized Losses | $ 0.0 |
Number of Securities | security | 5 |
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 29.7 |
Gross unrealized losses, less than twelve months | (0.5) |
Over 12 months, Fair Market Value | 0.0 |
Over 12 months, Gross Unrealized Losses | 0.0 |
Total, Fair Market Value | 29.7 |
Total, Gross Unrealized Losses | $ (0.5) |
Number of Securities | security | 20 |
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 804.6 | $ 39.6
Gross unrealized losses, less than twelve months | (11.7) | (0.1)
Over 12 months, Fair Market Value | 23.6 | 0.0
Over 12 months, Gross Unrealized Losses | (1.3) | 0.0
Total, Fair Market Value | 828.2 | 39.6
Total, Gross Unrealized Losses | $ (13.0) | $ (0.1)
Number of Securities | security | 400 | 16
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 114.4 |
Gross unrealized losses, less than twelve months | (1.9) |
Over 12 months, Fair Market Value | 0.0 |
Over 12 months, Gross Unrealized Losses | 0.0 |
Total, Fair Market Value | 114.4 |
Total, Gross Unrealized Losses | $ (1.9) |
Number of Securities | security | 10 |
Non-U.S government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 181.8 | $ 7.5
Gross unrealized losses, less than twelve months | (1.2) | 0.0
Over 12 months, Fair Market Value | 3.1 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 184.9 | 7.5
Total, Gross Unrealized Losses | $ (1.2) | $ 0.0
Number of Securities | security | 35 | 4
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 564.5 | $ 33.3
Gross unrealized losses, less than twelve months | (11.6) | (0.1)
Over 12 months, Fair Market Value | 16.0 | 2.1
Over 12 months, Gross Unrealized Losses | (0.4) | 0.0
Total, Fair Market Value | 580.5 | 35.4
Total, Gross Unrealized Losses | $ (12.0) | $ (0.1)
Number of Securities | security | 116 | 19

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