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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Trading Securities | $ 1,649.3 | $ 1,265.7
Fixed income securities — Trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1,634.9 | 1,264.8
Gross Unrealized Gains | 21.5 | 14.0
Gross Unrealized Losses | (7.1) | (13.1)
Trading Securities | 1,649.3 | 1,265.7
Short-term Investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 73.0 | 185.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Trading Securities | 73.0 | 185.4
Total equity securities — Trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 414.8 | 554.3
Gross Unrealized Gains | 83.5 | 55.4
Gross Unrealized Losses | (7.3) | (25.0)
Trading Securities | 491.0 | 584.7
Total catastrophe bonds — Trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 33.5 | 42.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (1.1) | 0.0
Trading Securities | 32.4 | 42.5
Total |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 2,156.2 | 2,047.0
Gross Unrealized Gains | 105.0 | 69.4
Gross Unrealized Losses | (15.5) | (38.1)
Trading Securities | 2,245.7 | 2,078.3
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 162.3 | 82.8
Gross Unrealized Gains | 0.4 | 0.4
Gross Unrealized Losses | (0.8) | (0.8)
Trading Securities | 161.9 | 82.4
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 32.4 | 15.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.2) | (0.2)
Trading Securities | 32.2 | 15.5
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1,036.5 | 817.8
Gross Unrealized Gains | 14.0 | 9.9
Gross Unrealized Losses | (4.2) | (7.1)
Trading Securities | 1,046.3 | 820.6
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1.0 |
Gross Unrealized Gains | 0.0 |
Gross Unrealized Losses | 0.0 |
Trading Securities | 1.0 |
Non-U.S government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 196.1 | 203.4
Gross Unrealized Gains | 6.9 | 3.5
Gross Unrealized Losses | (0.5) | (4.1)
Trading Securities | 202.5 | 202.8
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 9.9 | 14.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Trading Securities | 9.9 | 14.5
Agency mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 196.7 | 130.6
Gross Unrealized Gains | 0.2 | 0.2
Gross Unrealized Losses | (1.4) | (0.9)
Trading Securities | $ 195.5 | $ 129.9

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