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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 | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Statement [Line Items] |  |
Cost or Amortized Cost | $ 5,340.1 | $ 5,765.4
Gross Unrealized Gains | 81.9 | 85.3
Gross Unrealized Losses | (22.2) | (40.8)
Fair Market Value | 5,399.8 | 5,809.9
Total fixed income securities — Available for sale |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 5,305.7 | 5,620.1
Gross Unrealized Gains | 81.9 | 85.3
Gross Unrealized Losses | (22.2) | (40.8)
Fair Market Value | 5,365.4 | 5,664.6
Total short-term investments — Available for sale |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 34.4 | 145.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 34.4 | 145.3
U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,171.3 | 1,207.9
Gross Unrealized Gains | 7.6 | 9.4
Gross Unrealized Losses | (6.2) | (11.2)
Fair Market Value | 1,172.7 | 1,206.1
U.S. agency |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 61.3 | 117.7
Gross Unrealized Gains | 0.9 | 1.9
Gross Unrealized Losses | (0.1) | 0.0
Fair Market Value | 62.1 | 119.6
Municipal |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 47.9 | 23.2
Gross Unrealized Gains | 2.2 | 1.6
Gross Unrealized Losses | 0.0 | (0.4)
Fair Market Value | 50.1 | 24.4
Corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 2,431.5 | 2,566.9
Gross Unrealized Gains | 45.4 | 39.6
Gross Unrealized Losses | (8.6) | (20.0)
Fair Market Value | 2,468.3 | 2,586.5
Non-U.S. government-backed corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 94.0 | 89.2
Gross Unrealized Gains | 0.4 | 0.7
Gross Unrealized Losses | (0.4) | (0.1)
Fair Market Value | 94.0 | 89.8
Non-U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 525.3 | 477.7
Gross Unrealized Gains | 7.9 | 11.8
Gross Unrealized Losses | (1.5) | (0.8)
Fair Market Value | 531.7 | 488.7
Asset-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 33.6 | 62.6
Gross Unrealized Gains | 0.1 | 0.4
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 33.7 | 63.0
Non-agency commercial mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost |  | 12.3
Gross Unrealized Gains |  | 0.3
Gross Unrealized Losses |  | 0.0
Fair Market Value |  | 12.6
Agency mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 940.8 | 1,062.6
Gross Unrealized Gains | 17.4 | 19.6
Gross Unrealized Losses | (5.4) | (8.3)
Fair Market Value | $ 952.8 | $ 1,073.9

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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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