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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2018 USD ($) security Security | Dec. 31, 2017 USD ($) security Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,560.9 | $ 1,965.3
0-12 months, Gross Unrealized Losses | (19.1) | (12.0)
Over 12 months, Fair Market Value | 2,121.1 | 984.3
Over 12 months, Gross Unrealized Losses | (58.8) | (21.9)
Total, Fair Market Value | 3,682.0 | 2,949.6
Total, Gross Unrealized Losses | $ (77.9) | $ (33.9)
Number of Securities | security | 1,133 | 769
Fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,526.4 | $ 1,918.4
0-12 months, Gross Unrealized Losses | (19.1) | (11.9)
Over 12 months, Fair Market Value | 2,121.1 | 984.3
Over 12 months, Gross Unrealized Losses | (58.8) | (21.9)
Total, Fair Market Value | 3,647.5 | 2,902.7
Total, Gross Unrealized Losses | $ (77.9) | $ (33.8)
Number of Securities | Security | 1,121 | 761
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 34.5 | $ 46.9
0-12 months, Gross Unrealized Losses | 0.0 | (0.1)
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 34.5 | 46.9
Total, Gross Unrealized Losses | $ 0.0 | $ (0.1)
Number of Securities | Security | 12 | 8
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 180.2 | $ 652.1
0-12 months, Gross Unrealized Losses | (0.7) | (5.1)
Over 12 months, Fair Market Value | 740.6 | 259.8
Over 12 months, Gross Unrealized Losses | (15.4) | (6.5)
Total, Fair Market Value | 920.8 | 911.9
Total, Gross Unrealized Losses | $ (16.1) | $ (11.6)
Number of Securities | security | 103 | 101
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 13.5 | $ 20.1
0-12 months, Gross Unrealized Losses | (0.2) | (0.2)
Over 12 months, Fair Market Value | 18.4 | 6.1
Over 12 months, Gross Unrealized Losses | (0.2) | 0.0
Total, Fair Market Value | 31.9 | 26.2
Total, Gross Unrealized Losses | $ (0.4) | $ (0.2)
Number of Securities | security | 12 | 10
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 3.1 | $ 28.5
0-12 months, Gross Unrealized Losses | (0.1) | (0.2)
Over 12 months, Fair Market Value | 25.0 | 0.0
Over 12 months, Gross Unrealized Losses | (0.7) | 0.0
Total, Fair Market Value | 28.1 | 28.5
Total, Gross Unrealized Losses | $ (0.8) | $ (0.2)
Number of Securities | security | 9 | 9
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 999.1 | $ 699.3
0-12 months, Gross Unrealized Losses | (15.2) | (3.4)
Over 12 months, Fair Market Value | 762.2 | 360.7
Over 12 months, Gross Unrealized Losses | (25.3) | (8.4)
Total, Fair Market Value | 1,761.3 | 1,060.0
Total, Gross Unrealized Losses | $ (40.5) | $ (11.8)
Number of Securities | security | 667 | 412
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 14.5 | $ 43.5
0-12 months, Gross Unrealized Losses | 0.0 | (0.3)
Over 12 months, Fair Market Value | 25.8 | 13.3
Over 12 months, Gross Unrealized Losses | (0.2) | (0.2)
Total, Fair Market Value | 40.3 | 56.8
Total, Gross Unrealized Losses | $ (0.2) | $ (0.5)
Number of Securities | security | 12 | 15
Non-U.S government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 64.0 | $ 206.2
0-12 months, Gross Unrealized Losses | (0.3) | (0.8)
Over 12 months, Fair Market Value | 91.0 | 32.0
Over 12 months, Gross Unrealized Losses | (0.5) | (0.4)
Total, Fair Market Value | 155.0 | 238.2
Total, Gross Unrealized Losses | $ (0.8) | $ (1.2)
Number of Securities | security | 57 | 47
Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 6.3 | $ 11.1
0-12 months, Gross Unrealized Losses | 0.0 | 0.0
Over 12 months, Fair Market Value | 10.8 | 10.5
Over 12 months, Gross Unrealized Losses | (0.1) | (0.1)
Total, Fair Market Value | 17.1 | 21.6
Total, Gross Unrealized Losses | $ (0.1) | $ (0.1)
Number of Securities | security | 8 | 11
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 245.7 | $ 257.6
0-12 months, Gross Unrealized Losses | (2.6) | (1.9)
Over 12 months, Fair Market Value | 447.3 | 301.9
Over 12 months, Gross Unrealized Losses | (16.4) | (6.3)
Total, Fair Market Value | 693.0 | 559.5
Total, Gross Unrealized Losses | $ (19.0) | $ (8.2)
Number of Securities | security | 253 | 156

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