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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Mar. 31, 2018 USD ($) Security | Dec. 31, 2017 USD ($) Security
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 3,164.2 | $ 1,965.3
0-12 months, Gross Unrealized Loss | (50.6) | (12.0)
Over 12 months, Fair Market Value | 942.0 | 984.3
Over 12 months, Gross Unrealized Loss | (36.3) | (21.9)
Total, Fair Market Value | 4,106.2 | 2,949.6
Total, Gross Unrealized Loss | $ (86.9) | $ (33.9)
Number of Securities | Security | 1,116 | 769
Total fixed income securities — Available for sale |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 3,122.4 | $ 1,918.4
0-12 months, Gross Unrealized Loss | (50.5) | (11.9)
Over 12 months, Fair Market Value | 942.0 | 984.3
Over 12 months, Gross Unrealized Loss | (36.3) | (21.9)
Total, Fair Market Value | 4,064.4 | 2,902.7
Total, Gross Unrealized Loss | $ (86.8) | $ (33.8)
Number of Securities | Security | 1,106 | 761
Total short-term investments — Trading |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 41.8 | $ 46.9
0-12 months, Gross Unrealized Loss | (0.1) | (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 | 41.8 | 46.9
Total, Gross Unrealized Loss | $ (0.1) | $ (0.1)
Number of Securities | Security | 10 | 8
U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 772.9 | $ 652.1
0-12 months, Gross Unrealized Loss | (12.7) | (5.1)
Over 12 months, Fair Market Value | 286.8 | 259.8
Over 12 months, Gross Unrealized Loss | (8.9) | (6.5)
Total, Fair Market Value | 1,059.7 | 911.9
Total, Gross Unrealized Loss | $ (21.6) | $ (11.6)
Number of Securities | Security | 112 | 101
U.S. agency |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 30.8 | $ 20.1
0-12 months, Gross Unrealized Loss | (0.4) | (0.2)
Over 12 months, Fair Market Value | 5.5 | 6.1
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 36.3 | 26.2
Total, Gross Unrealized Loss | $ (0.4) | $ (0.2)
Number of Securities | Security | 12 | 10
Municipal |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 36.0 | $ 28.5
0-12 months, Gross Unrealized Loss | (0.6) | (0.2)
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 36.0 | 28.5
Total, Gross Unrealized Loss | $ (0.6) | $ (0.2)
Number of Securities | Security | 11 | 9
Corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 1,544.3 | $ 699.3
0-12 months, Gross Unrealized Loss | (26.3) | (3.4)
Over 12 months, Fair Market Value | 320.4 | 360.7
Over 12 months, Gross Unrealized Loss | (15.0) | (8.4)
Total, Fair Market Value | 1,864.7 | 1,060.0
Total, Gross Unrealized Loss | $ (41.3) | $ (11.8)
Number of Securities | Security | 661 | 412
Non-U.S. government-backed corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 50.0 | $ 43.5
0-12 months, Gross Unrealized Loss | (0.4) | (0.3)
Over 12 months, Fair Market Value | 10.0 | 13.3
Over 12 months, Gross Unrealized Loss | (0.1) | (0.2)
Total, Fair Market Value | 60.0 | 56.8
Total, Gross Unrealized Loss | $ (0.5) | $ (0.5)
Number of Securities | Security | 17 | 15
Non-U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 247.5 | $ 206.2
0-12 months, Gross Unrealized Loss | (1.4) | (0.8)
Over 12 months, Fair Market Value | 23.8 | 32.0
Over 12 months, Gross Unrealized Loss | (0.4) | (0.4)
Total, Fair Market Value | 271.3 | 238.2
Total, Gross Unrealized Loss | $ (1.8) | $ (1.2)
Number of Securities | Security | 58 | 47
Asset-backed |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 13.4 | $ 11.1
0-12 months, Gross Unrealized Loss | (0.1) | 0.0
Over 12 months, Fair Market Value | 5.0 | 10.5
Over 12 months, Gross Unrealized Loss | (0.1) | (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 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 427.5 | $ 257.6
0-12 months, Gross Unrealized Loss | (8.6) | (1.9)
Over 12 months, Fair Market Value | 290.5 | 301.9
Over 12 months, Gross Unrealized Loss | (11.8) | (6.3)
Total, Fair Market Value | 718.0 | 559.5
Total, Gross Unrealized Loss | $ (20.4) | $ (8.2)
Number of Securities | Security | 228 | 156

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