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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 | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Statement [Line Items] |  |
Fair Market Value | $ 1,313.2 | $ 1,265.7
U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 94.8 | 82.8
Gross Unrealized Gains | 0.7 | 0.4
Gross Unrealized Losses | (0.1) | (0.8)
Fair Market Value | 95.4 | 82.4
Municipal |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 15.7 | 15.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | (0.2)
Fair Market Value | 15.6 | 15.5
Corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 858.6 | 817.8
Gross Unrealized Gains | 12.4 | 9.9
Gross Unrealized Losses | (4.8) | (7.1)
Fair Market Value | 866.2 | 820.6
Non-U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 187.2 | 203.4
Gross Unrealized Gains | 5.7 | 3.5
Gross Unrealized Losses | (1.2) | (4.1)
Fair Market Value | 191.7 | 202.8
Asset-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 13.4 | 14.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 13.4 | 14.5
Agency mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 132.0 | 130.6
Gross Unrealized Gains | 0.1 | 0.2
Gross Unrealized Losses | (1.2) | (0.9)
Fair Market Value | 130.9 | 129.9
Total fixed income securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,301.7 | 1,264.8
Gross Unrealized Gains | 18.9 | 14.0
Gross Unrealized Losses | (7.4) | (13.1)
Fair Market Value | 1,313.2 | 1,265.7
Total short-term investments — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 184.7 | 185.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | 0.0
Fair Market Value | 184.6 | 185.4
Total equity securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 563.2 | 554.3
Gross Unrealized Gains | 78.3 | 55.4
Gross Unrealized Losses | (17.9) | (25.0)
Fair Market Value | 623.6 | 584.7
Total catastrophe bonds — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 41.8 | 42.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 41.8 | 42.5
Total |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 2,091.4 | 2,047.0
Gross Unrealized Gains | 97.2 | 69.4
Gross Unrealized Losses | (25.4) | (38.1)
Fair Market Value | $ 2,163.2 | $ 2,078.3

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