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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 | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Statement [Line Items] |  |
Fair Market Value | $ 1,357.5 | $ 1,265.7
U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 112.4 | 82.8
Gross Unrealized Gains | 0.7 | 0.4
Gross Unrealized Losses | (0.3) | (0.8)
Fair Market Value | 112.8 | 82.4
Municipal |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 5.7 | 15.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | (0.2)
Fair Market Value | 5.7 | 15.5
Corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 882.5 | 817.8
Gross Unrealized Gains | 16.1 | 9.9
Gross Unrealized Losses | (2.7) | (7.1)
Fair Market Value | 895.9 | 820.6
Non-U.S. government-backed corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 2.0 |
Gross Unrealized Gains | 0.0 |
Gross Unrealized Losses | 0.0 |
Fair Market Value | 2.0 |
Non-U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 180.8 | 203.4
Gross Unrealized Gains | 6.3 | 3.5
Gross Unrealized Losses | (0.6) | (4.1)
Fair Market Value | 186.5 | 202.8
Asset-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 12.2 | 14.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 12.2 | 14.5
Agency mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 142.7 | 130.6
Gross Unrealized Gains | 0.4 | 0.2
Gross Unrealized Losses | (0.7) | (0.9)
Fair Market Value | 142.4 | 129.9
Total fixed income securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,338.3 | 1,264.8
Gross Unrealized Gains | 23.5 | 14.0
Gross Unrealized Losses | (4.3) | (13.1)
Fair Market Value | 1,357.5 | 1,265.7
Total short-term investments — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 73.7 | 185.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 73.7 | 185.4
Total equity securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 569.5 | 554.3
Gross Unrealized Gains | 99.4 | 55.4
Gross Unrealized Losses | (10.2) | (25.0)
Fair Market Value | 658.7 | 584.7
Total catastrophe bonds — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 28.3 | 42.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 28.3 | 42.5
Total |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 2,009.8 | 2,047.0
Gross Unrealized Gains | 122.9 | 69.4
Gross Unrealized Losses | (14.5) | (38.1)
Fair Market Value | $ 2,118.2 | $ 2,078.3

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