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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, 2018 | Dec. 31, 2017
--- | --- | ---
Statement [Line Items] |  |
Cost or Amortized Cost | $ 1,644.6 | $ 1,634.9
Fair Market Value | 1,615.9 | 1,649.3
U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 155.6 | 162.3
Gross Unrealized Gains | 0.5 | 0.4
Gross Unrealized Losses | (0.8) | (0.8)
Fair Market Value | 155.3 | 161.9
Municipal |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 78.6 | 32.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.8) | (0.2)
Fair Market Value | 77.8 | 32.2
Corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 988.2 | 1,036.5
Gross Unrealized Gains | 3.1 | 14.0
Gross Unrealized Losses | (22.8) | (4.2)
Fair Market Value | 968.5 | 1,046.3
Non-U.S. government-backed corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 0.0 | 1.0
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 0.0 | 1.0
Non-U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 222.3 | 196.1
Gross Unrealized Gains | 2.0 | 6.9
Gross Unrealized Losses | (5.0) | (0.5)
Fair Market Value | 219.3 | 202.5
Asset-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 7.3 | 9.9
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | 0.0
Fair Market Value | 7.2 | 9.9
Agency mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 192.6 | 196.7
Gross Unrealized Gains | 0.0 | 0.2
Gross Unrealized Losses | (4.8) | (1.4)
Fair Market Value | 187.8 | 195.5
Total fixed income securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,644.6 | 1,634.9
Gross Unrealized Gains | 5.6 | 21.5
Gross Unrealized Losses | (34.3) | (7.1)
Fair Market Value | 1,615.9 | 1,649.3
Total short-term investments — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 27.4 | 73.0
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 27.4 | 73.0
Total equity securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost |  | 414.8
Gross Unrealized Gains |  | 83.5
Gross Unrealized Losses |  | (7.3)
Fair Market Value |  | 491.0
Total catastrophe bonds — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 35.8 | 33.5
Gross Unrealized Gains | 0.1 | 0.0
Gross Unrealized Losses | (0.4) | (1.1)
Fair Market Value | 35.5 | 32.4
Total |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,707.8 | 2,156.2
Gross Unrealized Gains | 5.7 | 105.0
Gross Unrealized Losses | (34.7) | (15.5)
Fair Market Value | $ 1,678.8 | $ 2,245.7

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