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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 | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Statement [Line Items] |  |
Fair Market Value | $ 1,665.0 | $ 1,265.7
U.S. government |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 160.6 | 82.8
Gross Unrealized Gains | 0.6 | 0.4
Gross Unrealized Losses | (0.6) | (0.8)
Fair Market Value | 160.6 | 82.4
Municipal |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 32.4 | 15.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | (0.2)
Fair Market Value | 32.3 | 15.5
Corporate |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,029.8 | 817.8
Gross Unrealized Gains | 17.9 | 9.9
Gross Unrealized Losses | (2.7) | (7.1)
Fair Market Value | 1,045.0 | 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 | 203.5 | 203.4
Gross Unrealized Gains | 7.8 | 3.5
Gross Unrealized Losses | (0.4) | (4.1)
Fair Market Value | 210.9 | 202.8
Asset-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 11.0 | 14.5
Gross Unrealized Gains | 0.1 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 11.1 | 14.5
Agency mortgage-backed |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 203.1 | 130.6
Gross Unrealized Gains | 0.7 | 0.2
Gross Unrealized Losses | (0.7) | (0.9)
Fair Market Value | 203.1 | 129.9
Total fixed income securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 1,642.4 | 1,264.8
Gross Unrealized Gains | 27.1 | 14.0
Gross Unrealized Losses | (4.5) | (13.1)
Fair Market Value | 1,665.0 | 1,265.7
Total short-term investments — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 90.3 | 185.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 90.3 | 185.4
Total equity securities — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 407.2 | 554.3
Gross Unrealized Gains | 68.7 | 55.4
Gross Unrealized Losses | (7.3) | (25.0)
Fair Market Value | 468.6 | 584.7
Total catastrophe bonds — Trading |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 33.5 | 42.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (3.2) | 0.0
Fair Market Value | 30.3 | 42.5
Total |  |
Statement [Line Items] |  |
Cost or Amortized Cost | 2,173.4 | 2,047.0
Gross Unrealized Gains | 95.8 | 69.4
Gross Unrealized Losses | (15.0) | (38.1)
Fair Market Value | $ 2,254.2 | $ 2,078.3

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