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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Statement [Line Items] |  |
Cost or Amortized Cost | $ 5,519.9 | $ 5,291.2
Gross Unrealized Gains | 33.9 | 63.6
Gross Unrealized Losses | (87.0) | (33.9)
Fair Market Value | 5,466.8 | 5,320.9
Total fixed income securities — Available for sale |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 5,465.2 | 5,201.2
Gross Unrealized Gains | 33.9 | 63.6
Gross Unrealized Losses | (86.9) | (33.8)
Fair Market Value | 5,412.2 | 5,231.0
Total short-term investments — Available for sale |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 54.7 | 90.0
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | (0.1)
Fair Market Value | 54.6 | 89.9
U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,311.3 | 1,166.5
Gross Unrealized Gains | 2.5 | 4.5
Gross Unrealized Losses | (21.6) | (11.6)
Fair Market Value | 1,292.2 | 1,159.4
U.S. agency |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 52.9 | 51.8
Gross Unrealized Gains | 0.2 | 0.5
Gross Unrealized Losses | (0.4) | (0.2)
Fair Market Value | 52.7 | 52.1
Municipal |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 46.5 | 53.0
Gross Unrealized Gains | 1.7 | 2.1
Gross Unrealized Losses | (0.6) | (0.2)
Fair Market Value | 47.6 | 54.9
Corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 2,471.3 | 2,391.4
Gross Unrealized Gains | 16.3 | 36.1
Gross Unrealized Losses | (41.4) | (11.8)
Fair Market Value | 2,446.2 | 2,415.7
Non-U.S. government-backed corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 98.2 | 91.5
Gross Unrealized Gains | 0.1 | 0.3
Gross Unrealized Losses | (0.5) | (0.5)
Fair Market Value | 97.8 | 91.3
Non-U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 480.7 | 479.7
Gross Unrealized Gains | 4.5 | 6.4
Gross Unrealized Losses | (1.8) | (1.2)
Fair Market Value | 483.4 | 484.9
Asset-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 18.7 | 26.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.2) | (0.1)
Fair Market Value | 18.5 | 26.2
Agency mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 985.6 | 941.0
Gross Unrealized Gains | 8.6 | 13.7
Gross Unrealized Losses | (20.4) | (8.2)
Fair Market Value | $ 973.8 | $ 946.5

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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