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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Sep. 30, 2018 USD ($) Security | Dec. 31, 2017 USD ($) Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 3,106.3 | $ 1,965.3
0-12 months, Gross Unrealized Loss | (57.1) | (12.0)
Over 12 months, Fair Market Value | 1,334.8 | 984.3
Over 12 months, Gross Unrealized Loss | (58.0) | (21.9)
Total, Fair Market Value | 4,441.1 | 2,949.6
Total, Gross Unrealized Loss | $ (115.1) | $ (33.9)
Number of Securities | Security | 1,254 | 769
Total fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 3,029.9 | $ 1,918.4
0-12 months, Gross Unrealized Loss | (57.1) | (11.9)
Over 12 months, Fair Market Value | 1,334.8 | 984.3
Over 12 months, Gross Unrealized Loss | (58.0) | (21.9)
Total, Fair Market Value | 4,364.7 | 2,902.7
Total, Gross Unrealized Loss | $ (115.1) | $ (33.8)
Number of Securities | Security | 1,241 | 761
Total short-term investments — Trading |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 76.4 | $ 46.9
0-12 months, Gross Unrealized Loss | 0.0 | (0.1)
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 76.4 | 46.9
Total, Gross Unrealized Loss | $ 0.0 | $ (0.1)
Number of Securities | Security | 13 | 8
U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 827.9 | $ 652.1
0-12 months, Gross Unrealized Loss | (15.3) | (5.1)
Over 12 months, Fair Market Value | 457.3 | 259.8
Over 12 months, Gross Unrealized Loss | (17.8) | (6.5)
Total, Fair Market Value | 1,285.2 | 911.9
Total, Gross Unrealized Loss | $ (33.1) | $ (11.6)
Number of Securities | Security | 140 | 101
U.S. agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 25.5 | $ 20.1
0-12 months, Gross Unrealized Loss | (0.4) | (0.2)
Over 12 months, Fair Market Value | 13.1 | 6.1
Over 12 months, Gross Unrealized Loss | (0.4) | 0.0
Total, Fair Market Value | 38.6 | 26.2
Total, Gross Unrealized Loss | $ (0.8) | $ (0.2)
Number of Securities | Security | 15 | 10
Municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 33.3 | $ 28.5
0-12 months, Gross Unrealized Loss | (1.1) | (0.2)
Over 12 months, Fair Market Value | 7.2 | 0.0
Over 12 months, Gross Unrealized Loss | (0.4) | 0.0
Total, Fair Market Value | 40.5 | 28.5
Total, Gross Unrealized Loss | $ (1.5) | $ (0.2)
Number of Securities | Security | 13 | 9
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 1,466.5 | $ 699.3
0-12 months, Gross Unrealized Loss | (28.7) | (3.4)
Over 12 months, Fair Market Value | 416.7 | 360.7
Over 12 months, Gross Unrealized Loss | (19.2) | (8.4)
Total, Fair Market Value | 1,883.2 | 1,060.0
Total, Gross Unrealized Loss | $ (47.9) | $ (11.8)
Number of Securities | Security | 725 | 412
Non-U.S. government-backed corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 43.2 | $ 43.5
0-12 months, Gross Unrealized Loss | (0.3) | (0.3)
Over 12 months, Fair Market Value | 25.4 | 13.3
Over 12 months, Gross Unrealized Loss | (0.4) | (0.2)
Total, Fair Market Value | 68.6 | 56.8
Total, Gross Unrealized Loss | $ (0.7) | $ (0.5)
Number of Securities | Security | 15 | 15
Non-U.S. government |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 193.8 | $ 206.2
0-12 months, Gross Unrealized Loss | (0.9) | (0.8)
Over 12 months, Fair Market Value | 50.1 | 32.0
Over 12 months, Gross Unrealized Loss | (0.9) | (0.4)
Total, Fair Market Value | 243.9 | 238.2
Total, Gross Unrealized Loss | $ (1.8) | $ (1.2)
Number of Securities | Security | 61 | 47
Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 13.4 | $ 11.1
0-12 months, Gross Unrealized Loss | (0.2) | 0.0
Over 12 months, Fair Market Value | 5.0 | 10.5
Over 12 months, Gross Unrealized Loss | 0.0 | (0.1)
Total, Fair Market Value | 18.4 | 21.6
Total, Gross Unrealized Loss | $ (0.2) | $ (0.1)
Number of Securities | Security | 7 | 11
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
0-12 months, Fair Market Value | $ 426.3 | $ 257.6
0-12 months, Gross Unrealized Loss | (10.2) | (1.9)
Over 12 months, Fair Market Value | 360.0 | 301.9
Over 12 months, Gross Unrealized Loss | (18.9) | (6.3)
Total, Fair Market Value | 786.3 | 559.5
Total, Gross Unrealized Loss | $ (29.1) | $ (8.2)
Number of Securities | Security | 265 | 156

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