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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Jun. 30, 2018 USD ($) Security | Dec. 31, 2017 USD ($) Security
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Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 3,181.3 | $ 1,965.3
0-12 months, Gross Unrealized Loss | (60.3) | (12.0)
Over 12 months, Fair Market Value | 916.7 | 984.3
Over 12 months, Gross Unrealized Loss | (39.1) | (21.9)
Total, Fair Market Value | 4,098.0 | 2,949.6
Total, Gross Unrealized Loss | $ (99.4) | $ (33.9)
Number of Securities | Security | 1,157 | 769
Total fixed income securities — Available for sale |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 3,139.8 | $ 1,918.4
0-12 months, Gross Unrealized Loss | (60.3) | (11.9)
Over 12 months, Fair Market Value | 916.7 | 984.3
Over 12 months, Gross Unrealized Loss | (39.1) | (21.9)
Total, Fair Market Value | 4,056.5 | 2,902.7
Total, Gross Unrealized Loss | $ (99.4) | $ (33.8)
Number of Securities | Security | 1,149 | 761
Total short-term investments — Trading |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 41.5 | $ 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 | 41.5 | 46.9
Total, Gross Unrealized Loss | $ 0.0 | $ (0.1)
Number of Securities | Security | 8 | 8
U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 808.1 | $ 652.1
0-12 months, Gross Unrealized Loss | (15.8) | (5.1)
Over 12 months, Fair Market Value | 256.1 | 259.8
Over 12 months, Gross Unrealized Loss | (9.7) | (6.5)
Total, Fair Market Value | 1,064.2 | 911.9
Total, Gross Unrealized Loss | $ (25.5) | $ (11.6)
Number of Securities | Security | 123 | 101
U.S. agency |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 32.9 | $ 20.1
0-12 months, Gross Unrealized Loss | (0.6) | (0.2)
Over 12 months, Fair Market Value | 0.0 | 6.1
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 32.9 | 26.2
Total, Gross Unrealized Loss | $ (0.6) | $ (0.2)
Number of Securities | Security | 13 | 10
Municipal |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 90.0 | $ 28.5
0-12 months, Gross Unrealized Loss | (1.1) | (0.2)
Over 12 months, Fair Market Value | 0.7 | 0.0
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 90.7 | 28.5
Total, Gross Unrealized Loss | $ (1.1) | $ (0.2)
Number of Securities | Security | 13 | 9
Corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 1,556.6 | $ 699.3
0-12 months, Gross Unrealized Loss | (31.9) | (3.4)
Over 12 months, Fair Market Value | 285.3 | 360.7
Over 12 months, Gross Unrealized Loss | (15.4) | (8.4)
Total, Fair Market Value | 1,841.9 | 1,060.0
Total, Gross Unrealized Loss | $ (47.3) | $ (11.8)
Number of Securities | Security | 682 | 412
Non-U.S. government-backed corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 42.9 | $ 43.5
0-12 months, Gross Unrealized Loss | (0.2) | (0.3)
Over 12 months, Fair Market Value | 20.9 | 13.3
Over 12 months, Gross Unrealized Loss | (0.3) | (0.2)
Total, Fair Market Value | 63.8 | 56.8
Total, Gross Unrealized Loss | $ (0.5) | $ (0.5)
Number of Securities | Security | 14 | 15
Non-U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 188.9 | $ 206.2
0-12 months, Gross Unrealized Loss | (0.8) | (0.8)
Over 12 months, Fair Market Value | 59.3 | 32.0
Over 12 months, Gross Unrealized Loss | (0.5) | (0.4)
Total, Fair Market Value | 248.2 | 238.2
Total, Gross Unrealized Loss | $ (1.3) | $ (1.2)
Number of Securities | Security | 57 | 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 | (0.2) | 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 | $ (0.3) | $ (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 | $ 407.0 | $ 257.6
0-12 months, Gross Unrealized Loss | (9.7) | (1.9)
Over 12 months, Fair Market Value | 289.4 | 301.9
Over 12 months, Gross Unrealized Loss | (13.1) | (6.3)
Total, Fair Market Value | 696.4 | 559.5
Total, Gross Unrealized Loss | $ (22.8) | $ (8.2)
Number of Securities | Security | 240 | 156

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