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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, 2018 | Dec. 31, 2017
--- | --- | ---
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 5,393.8 | $ 5,291.2
Gross Unrealized Gains | 18.1 | 63.6
Gross Unrealized Losses | (115.1) | (33.9)
Fair Market Value | 5,296.8 | 5,320.9
Total fixed income securities — Available for sale |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,273.4 | 5,201.2
Gross Unrealized Gains | 18.1 | 63.6
Gross Unrealized Losses | (115.1) | (33.8)
Fair Market Value | 5,176.4 | 5,231.0
Total short-term investments — Available for sale |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 120.4 | 90.0
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | (0.1)
Fair Market Value | 120.4 | 89.9
U.S. government |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 1,420.1 | 1,166.5
Gross Unrealized Gains | 0.8 | 4.5
Gross Unrealized Losses | (33.1) | (11.6)
Fair Market Value | 1,387.8 | 1,159.4
U.S. agency |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 47.1 | 51.8
Gross Unrealized Gains | 0.1 | 0.5
Gross Unrealized Losses | (0.8) | (0.2)
Fair Market Value | 46.4 | 52.1
Municipal |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 51.9 | 53.0
Gross Unrealized Gains | 1.7 | 2.1
Gross Unrealized Losses | (1.5) | (0.2)
Fair Market Value | 52.1 | 54.9
Corporate |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 2,305.9 | 2,391.4
Gross Unrealized Gains | 8.0 | 36.1
Gross Unrealized Losses | (47.9) | (11.8)
Fair Market Value | 2,266.0 | 2,415.7
Non-U.S. government-backed corporate |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 91.4 | 91.5
Gross Unrealized Gains | 0.1 | 0.3
Gross Unrealized Losses | (0.7) | (0.5)
Fair Market Value | 90.8 | 91.3
Non-U.S. government |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 404.1 | 479.7
Gross Unrealized Gains | 3.1 | 6.4
Gross Unrealized Losses | (1.8) | (1.2)
Fair Market Value | 405.4 | 484.9
Asset-backed |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, 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] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 934.2 | 941.0
Gross Unrealized Gains | 4.3 | 13.7
Gross Unrealized Losses | (29.1) | (8.2)
Fair Market Value | $ 909.4 | $ 946.5

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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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