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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions | Dec. 31, 2017 USD ($) security Security | Dec. 31, 2016 USD ($) security Security
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 1,965.3 | $ 2,518.2
0-12 months, Gross Unrealized Losses | (12.0) | (39.3)
Over 12 months, Fair Market Value | 984.3 | 37.0
Over 12 months, Gross Unrealized Losses | (21.9) | (1.5)
Total, Fair Market Value | 2,949.6 | 2,555.2
Total, Gross Unrealized Losses | $ (33.9) | $ (40.8)
Number of Securities | security | 769 | 679
Fixed income securities — Available for sale |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 1,918.4 | $ 2,517.1
0-12 months, Gross Unrealized Losses | (11.9) | (39.3)
Over 12 months, Fair Market Value | 984.3 | 37.0
Over 12 months, Gross Unrealized Losses | (21.9) | (1.5)
Total, Fair Market Value | 2,902.7 | 2,554.1
Total, Gross Unrealized Losses | $ (33.8) | $ (40.8)
Number of Securities | Security | 761 | 677
Short-term Investments |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 46.9 | $ 1.1
0-12 months, Gross Unrealized Losses | (0.1) | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 46.9 | 1.1
Total, Gross Unrealized Losses | $ (0.1) | $ 0.0
Number of Securities | Security | 8 | 2
U.S. government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 652.1 | $ 724.4
0-12 months, Gross Unrealized Losses | (5.1) | (11.2)
Over 12 months, Fair Market Value | 259.8 | 0.0
Over 12 months, Gross Unrealized Losses | (6.5) | 0.0
Total, Fair Market Value | 911.9 | 724.4
Total, Gross Unrealized Losses | $ (11.6) | $ (11.2)
Number of Securities | security | 101 | 78
U.S. agency |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 20.1 | $ 14.1
0-12 months, Gross Unrealized Losses | (0.2) | 0.0
Over 12 months, Fair Market Value | 6.1 | 0.0
Over 12 months, Gross Unrealized Losses | 0.0 | 0.0
Total, Fair Market Value | 26.2 | 14.1
Total, Gross Unrealized Losses | $ (0.2) | $ 0.0
Number of Securities | security | 10 | 4
Municipal |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 28.5 | $ 7.7
0-12 months, Gross Unrealized Losses | (0.2) | (0.2)
Over 12 months, Fair Market Value | 0.0 | 0.8
Over 12 months, Gross Unrealized Losses | 0.0 | (0.2)
Total, Fair Market Value | 28.5 | 8.5
Total, Gross Unrealized Losses | $ (0.2) | $ (0.4)
Number of Securities | security | 9 | 6
Corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 699.3 | $ 1,044.4
0-12 months, Gross Unrealized Losses | (3.4) | (19.4)
Over 12 months, Fair Market Value | 360.7 | 6.6
Over 12 months, Gross Unrealized Losses | (8.4) | (0.6)
Total, Fair Market Value | 1,060.0 | 1,051.0
Total, Gross Unrealized Losses | $ (11.8) | $ (20.0)
Number of Securities | security | 412 | 386
Non-U.S. government-backed corporate |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 43.5 | $ 29.6
0-12 months, Gross Unrealized Losses | (0.3) | (0.1)
Over 12 months, Fair Market Value | 13.3 | 0.0
Over 12 months, Gross Unrealized Losses | (0.2) | 0.0
Total, Fair Market Value | 56.8 | 29.6
Total, Gross Unrealized Losses | $ (0.5) | $ (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 | $ 206.2 | $ 143.5
0-12 months, Gross Unrealized Losses | (0.8) | (0.8)
Over 12 months, Fair Market Value | 32.0 | 1.0
Over 12 months, Gross Unrealized Losses | (0.4) | 0.0
Total, Fair Market Value | 238.2 | 144.5
Total, Gross Unrealized Losses | $ (1.2) | $ (0.8)
Number of Securities | security | 47 | 29
Asset-backed |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 11.1 | $ 25.8
0-12 months, Gross Unrealized Losses | 0.0 | 0.0
Over 12 months, Fair Market Value | 10.5 | 1.4
Over 12 months, Gross Unrealized Losses | (0.1) | 0.0
Total, Fair Market Value | 21.6 | 27.2
Total, Gross Unrealized Losses | $ (0.1) | $ 0.0
Number of Securities | security | 11 | 15
Agency mortgage-backed |  |
Schedule of Available-for-sale Securities [Line Items] |  |
0-12 months, Fair Market Value | $ 257.6 | $ 527.6
0-12 months, Gross Unrealized Losses | (1.9) | (7.6)
Over 12 months, Fair Market Value | 301.9 | 27.2
Over 12 months, Gross Unrealized Losses | (6.3) | (0.7)
Total, Fair Market Value | 559.5 | 554.8
Total, Gross Unrealized Losses | $ (8.2) | $ (8.3)
Number of Securities | security | 156 | 148

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