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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2020 USD ($) security Security | Dec. 31, 2019 USD ($) security Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 147.8 | $ 582.5
0-12 months, Gross Unrealized Losses | (0.8) | (2.6)
Over 12 months, Fair Market Value | 2.1 | 250.5
Over 12 months, Gross Unrealized Losses | (0.2) | (1.2)
Total, Fair Market Value | 149.9 | 833.0
Total, Gross Unrealized Losses | $ (1.0) | $ (3.8)
Number of Securities | security | 64 | 260
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 139.1 | $ 553.0
0-12 months, Gross Unrealized Losses | (0.8) | (2.6)
Over 12 months, Fair Market Value | 2.1 | 250.5
Over 12 months, Gross Unrealized Losses | 0.0 | (1.2)
Total, Fair Market Value | 141.2 | 803.5
Total, Gross Unrealized Losses | $ (0.8) | $ (3.8)
Number of Securities | Security | 49 | 255
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 8.7 | $ 29.5
0-12 months, Gross Unrealized Losses | 0.0 | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Losses | (0.2) | 0.0
Total, Fair Market Value | 8.7 | 29.5
Total, Gross Unrealized Losses | $ (0.2) | $ 0.0
Number of Securities | 15 | 5
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 58.7 | $ 142.0
0-12 months, Gross Unrealized Losses | (0.6) | (1.0)
Over 12 months, Fair Market Value | 0.0 | 138.2
Over 12 months, Gross Unrealized Losses | 0.0 | (0.4)
Total, Fair Market Value | 58.7 | 280.2
Total, Gross Unrealized Losses | $ (0.6) | $ (1.4)
Number of Securities | security | 10 | 49
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 3.0
0-12 months, Gross Unrealized Losses |  | 0.0
Over 12 months, Fair Market Value |  | 6.0
Over 12 months, Gross Unrealized Losses |  | 0.0
Total, Fair Market Value |  | 9.0
Total, Gross Unrealized Losses |  | $ 0.0
Number of Securities | security |  | 2
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 3.7
0-12 months, Gross Unrealized Losses |  | 0.0
Over 12 months, Fair Market Value |  | 0.0
Over 12 months, Gross Unrealized Losses |  | 0.0
Total, Fair Market Value |  | 3.7
Total, Gross Unrealized Losses |  | $ 0.0
Number of Securities | security |  | 2
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 39.6 | $ 167.7
0-12 months, Gross Unrealized Losses | (0.1) | (0.6)
Over 12 months, Fair Market Value | 0.0 | 37.1
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 39.6 | 204.8
Total, Gross Unrealized Losses | $ (0.1) | $ (0.6)
Number of Securities | security | 16 | 91
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 31.8
0-12 months, Gross Unrealized Losses |  | (0.1)
Over 12 months, Fair Market Value |  | 0.0
Over 12 months, Gross Unrealized Losses |  | 0.0
Total, Fair Market Value |  | 31.8
Total, Gross Unrealized Losses |  | $ (0.1)
Number of Securities | security |  | 9
Non-U.S government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 7.5 | $ 48.6
0-12 months, Gross Unrealized Losses | 0.0 | (0.4)
Over 12 months, Fair Market Value | 0.0 | 0.6
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 7.5 | 49.2
Total, Gross Unrealized Losses | $ 0.0 | $ (0.4)
Number of Securities | security | 4 | 20
Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 0.0
0-12 months, Gross Unrealized Losses |  | 0.0
Over 12 months, Fair Market Value |  | 0.2
Over 12 months, Gross Unrealized Losses |  | 0.0
Total, Fair Market Value |  | 0.2
Total, Gross Unrealized Losses |  | $ 0.0
Number of Securities | security |  | 1
Non-agency commercial mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value |  | $ 6.5
0-12 months, Gross Unrealized Losses |  | (0.2)
Over 12 months, Fair Market Value |  | 0.0
Over 12 months, Gross Unrealized Losses |  | 0.0
Total, Fair Market Value |  | 6.5
Total, Gross Unrealized Losses |  | $ (0.2)
Number of Securities | security |  | 1
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 33.3 | $ 149.7
0-12 months, Gross Unrealized Losses | (0.1) | (0.3)
Over 12 months, Fair Market Value | 2.1 | 68.4
Over 12 months, Gross Unrealized Losses | 0.0 | (0.8)
Total, Fair Market Value | 35.4 | 218.1
Total, Gross Unrealized Losses | $ (0.1) | $ (1.1)
Number of Securities | security | 19 | 80

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