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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 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | $ 5,291.2 | $ 5,765.4
Gross Unrealized Gains | 63.6 | 85.3
Gross Unrealized Losses | (33.9) | (40.8)
Fair Market Value | 5,320.9 | 5,809.9
Total fixed income securities — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 5,201.2 | 5,620.1
Gross Unrealized Gains | 63.6 | 85.3
Gross Unrealized Losses | (33.8) | (40.8)
Fair Market Value | 5,231.0 | 5,664.6
Total short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 90.0 | 145.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | 0.0
Fair Market Value | 89.9 | 145.3
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1,166.5 | 1,207.9
Gross Unrealized Gains | 4.5 | 9.4
Gross Unrealized Losses | (11.6) | (11.2)
Fair Market Value | 1,159.4 | 1,206.1
U.S. agency |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 51.8 | 117.7
Gross Unrealized Gains | 0.5 | 1.9
Gross Unrealized Losses | (0.2) | 0.0
Fair Market Value | 52.1 | 119.6
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 53.0 | 23.2
Gross Unrealized Gains | 2.1 | 1.6
Gross Unrealized Losses | (0.2) | (0.4)
Fair Market Value | 54.9 | 24.4
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 2,391.4 | 2,566.9
Gross Unrealized Gains | 36.1 | 39.6
Gross Unrealized Losses | (11.8) | (20.0)
Fair Market Value | 2,415.7 | 2,586.5
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 91.5 | 89.2
Gross Unrealized Gains | 0.3 | 0.7
Gross Unrealized Losses | (0.5) | (0.1)
Fair Market Value | 91.3 | 89.8
Non-U.S government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 479.7 | 477.7
Gross Unrealized Gains | 6.4 | 11.8
Gross Unrealized Losses | (1.2) | (0.8)
Fair Market Value | 484.9 | 488.7
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 26.3 | 62.6
Gross Unrealized Gains | 0.0 | 0.4
Gross Unrealized Losses | (0.1) | 0.0
Fair Market Value | 26.2 | 63.0
Non-agency commercial mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 0.0 | 12.3
Gross Unrealized Gains | 0.0 | 0.3
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 0.0 | 12.6
Agency mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 941.0 | 1,062.6
Gross Unrealized Gains | 13.7 | 19.6
Gross Unrealized Losses | (8.2) | (8.3)
Fair Market Value | $ 946.5 | $ 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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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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