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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Sep. 30, 2017 USD ($) Security | Dec. 31, 2016 USD ($) Security
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 1,879.0 | $ 2,518.2
0-12 months, Gross Unrealized Loss | (15.8) | (39.3)
Over 12 months, Fair Market Value | 287.7 | 37.0
Over 12 months, Gross Unrealized Loss | (6.4) | (1.5)
Total, Fair Market Value | 2,166.7 | 2,555.2
Total, Gross Unrealized Loss | $ (22.2) | $ (40.8)
Number of Securities | Security | 558 | 679
Total fixed income securities — Available for sale |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 1,853.3 | $ 2,517.1
0-12 months, Gross Unrealized Loss | (15.8) | (39.3)
Over 12 months, Fair Market Value | 287.7 | 37.0
Over 12 months, Gross Unrealized Loss | (6.4) | (1.5)
Total, Fair Market Value | 2,141.0 | 2,554.1
Total, Gross Unrealized Loss | $ (22.2) | $ (40.8)
Number of Securities | Security | 548 | 677
Total short-term investments — Trading |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 25.7 | $ 1.1
0-12 months, Gross Unrealized Loss | 0.0 | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 25.7 | 1.1
Total, Gross Unrealized Loss | $ 0.0 | $ 0.0
Number of Securities | Security | 10 | 2
U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 617.5 | $ 724.4
0-12 months, Gross Unrealized Loss | (5.0) | (11.2)
Over 12 months, Fair Market Value | 62.5 | 0.0
Over 12 months, Gross Unrealized Loss | (1.2) | 0.0
Total, Fair Market Value | 680.0 | 724.4
Total, Gross Unrealized Loss | $ (6.2) | $ (11.2)
Number of Securities | Security | 80 | 78
U.S. agency |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 19.6 | $ 14.1
0-12 months, Gross Unrealized Loss | (0.1) | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 19.6 | 14.1
Total, Gross Unrealized Loss | $ (0.1) | $ 0.0
Number of Securities | Security | 7 | 4
Municipal |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 19.2 | $ 7.7
0-12 months, Gross Unrealized Loss | 0.0 | (0.2)
Over 12 months, Fair Market Value | 0.0 | 0.8
Over 12 months, Gross Unrealized Loss | 0.0 | (0.2)
Total, Fair Market Value | 19.2 | 8.5
Total, Gross Unrealized Loss | $ 0.0 | $ (0.4)
Number of Securities | Security | 6 | 6
Corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 483.9 | $ 1,044.4
0-12 months, Gross Unrealized Loss | (4.6) | (19.4)
Over 12 months, Fair Market Value | 154.1 | 6.6
Over 12 months, Gross Unrealized Loss | (4.0) | (0.6)
Total, Fair Market Value | 638.0 | 1,051.0
Total, Gross Unrealized Loss | $ (8.6) | $ (20.0)
Number of Securities | Security | 255 | 386
Non-U.S. government-backed corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 63.4 | $ 29.6
0-12 months, Gross Unrealized Loss | (0.4) | (0.1)
Over 12 months, Fair Market Value | 0.4 | 0.0
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 63.8 | 29.6
Total, Gross Unrealized Loss | $ (0.4) | $ (0.1)
Number of Securities | Security | 15 | 11
Non-U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 211.5 | $ 143.5
0-12 months, Gross Unrealized Loss | (1.2) | (0.8)
Over 12 months, Fair Market Value | 36.8 | 1.0
Over 12 months, Gross Unrealized Loss | (0.3) | 0.0
Total, Fair Market Value | 248.3 | 144.5
Total, Gross Unrealized Loss | $ (1.5) | $ (0.8)
Number of Securities | Security | 43 | 29
Asset-backed |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 12.2 | $ 25.8
0-12 months, Gross Unrealized Loss | 0.0 | 0.0
Over 12 months, Fair Market Value | 0.0 | 1.4
Over 12 months, Gross Unrealized Loss | 0.0 | 0.0
Total, Fair Market Value | 12.2 | 27.2
Total, Gross Unrealized Loss | $ 0.0 | $ 0.0
Number of Securities | Security | 9 | 15
Agency mortgage-backed |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 426.0 | $ 527.6
0-12 months, Gross Unrealized Loss | (4.5) | (7.6)
Over 12 months, Fair Market Value | 33.9 | 27.2
Over 12 months, Gross Unrealized Loss | (0.9) | (0.7)
Total, Fair Market Value | 459.9 | 554.8
Total, Gross Unrealized Loss | $ (5.4) | $ (8.3)
Number of Securities | Security | 133 | 148

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