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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2019 USD ($) security Security | Dec. 31, 2018 USD ($) security Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 582.5 | $ 1,560.9
0-12 months, Gross Unrealized Losses | (2.6) | (19.1)
Over 12 months, Fair Market Value | 250.5 | 2,121.1
Over 12 months, Gross Unrealized Losses | (1.2) | (58.8)
Total, Fair Market Value | 833.0 | 3,682.0
Total, Gross Unrealized Losses | $ (3.8) | $ (77.9)
Number of Securities | security | 260 | 1,133
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 553.0 | $ 1,526.4
0-12 months, Gross Unrealized Losses | (2.6) | (19.1)
Over 12 months, Fair Market Value | 250.5 | 2,121.1
Over 12 months, Gross Unrealized Losses | (1.2) | (58.8)
Total, Fair Market Value | 803.5 | 3,647.5
Total, Gross Unrealized Losses | $ (3.8) | $ (77.9)
Number of Securities | Security | 255 | 1,121
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 29.5 | $ 34.5
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 | 0.0 | 0.0
Total, Fair Market Value | 29.5 | 34.5
Total, Gross Unrealized Losses | $ 0.0 | $ 0.0
Number of Securities | 5 | 12
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 142.0 | $ 180.2
0-12 months, Gross Unrealized Losses | (1.0) | (0.7)
Over 12 months, Fair Market Value | 138.2 | 740.6
Over 12 months, Gross Unrealized Losses | (0.4) | (15.4)
Total, Fair Market Value | 280.2 | 920.8
Total, Gross Unrealized Losses | $ (1.4) | $ (16.1)
Number of Securities | security | 49 | 103
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 3.0 | $ 13.5
0-12 months, Gross Unrealized Losses | 0.0 | (0.2)
Over 12 months, Fair Market Value | 6.0 | 18.4
Over 12 months, Gross Unrealized Losses | 0.0 | (0.2)
Total, Fair Market Value | 9.0 | 31.9
Total, Gross Unrealized Losses | $ 0.0 | $ (0.4)
Number of Securities | security | 2 | 12
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 3.7 | $ 3.1
0-12 months, Gross Unrealized Losses | 0.0 | (0.1)
Over 12 months, Fair Market Value | 0.0 | 25.0
Over 12 months, Gross Unrealized Losses | 0.0 | (0.7)
Total, Fair Market Value | 3.7 | 28.1
Total, Gross Unrealized Losses | $ 0.0 | $ (0.8)
Number of Securities | security | 2 | 9
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 167.7 | $ 999.1
0-12 months, Gross Unrealized Losses | (0.6) | (15.2)
Over 12 months, Fair Market Value | 37.1 | 762.2
Over 12 months, Gross Unrealized Losses | 0.0 | (25.3)
Total, Fair Market Value | 204.8 | 1,761.3
Total, Gross Unrealized Losses | $ (0.6) | $ (40.5)
Number of Securities | security | 91 | 667
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 31.8 | $ 14.5
0-12 months, Gross Unrealized Losses | (0.1) | 0.0
Over 12 months, Fair Market Value | 0.0 | 25.8
Over 12 months, Gross Unrealized Losses | 0.0 | (0.2)
Total, Fair Market Value | 31.8 | 40.3
Total, Gross Unrealized Losses | $ (0.1) | $ (0.2)
Number of Securities | security | 9 | 12
Non-U.S government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 48.6 | $ 64.0
0-12 months, Gross Unrealized Losses | (0.4) | (0.3)
Over 12 months, Fair Market Value | 0.6 | 91.0
Over 12 months, Gross Unrealized Losses | 0.0 | (0.5)
Total, Fair Market Value | 49.2 | 155.0
Total, Gross Unrealized Losses | $ (0.4) | $ (0.8)
Number of Securities | security | 20 | 57
Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 0.0 | $ 6.3
0-12 months, Gross Unrealized Losses | 0.0 | 0.0
Over 12 months, Fair Market Value | 0.2 | 10.8
Over 12 months, Gross Unrealized Losses | 0.0 | (0.1)
Total, Fair Market Value | 0.2 | 17.1
Total, Gross Unrealized Losses | $ 0.0 | $ (0.1)
Number of Securities | security | 1 | 8
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 | $ 149.7 | $ 245.7
0-12 months, Gross Unrealized Losses | (0.3) | (2.6)
Over 12 months, Fair Market Value | 68.4 | 447.3
Over 12 months, Gross Unrealized Losses | (0.8) | (16.4)
Total, Fair Market Value | 218.1 | 693.0
Total, Gross Unrealized Losses | $ (1.1) | $ (19.0)
Number of Securities | security | 80 | 253

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