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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, 2018
Dec. 31, 2017
Statement [Line Items]    
Cost or Amortized Cost $ 1,434.2 $ 1,634.9
Fair Market Value 1,407.2 1,649.3
U.S. government    
Statement [Line Items]    
Cost or Amortized Cost 167.2 162.3
Gross Unrealized Gains 0.0 0.4
Gross Unrealized Losses (1.3) (0.8)
Fair Market Value 165.9 161.9
Municipal    
Statement [Line Items]    
Cost or Amortized Cost 21.7 32.4
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (1.0) (0.2)
Fair Market Value 20.7 32.2
Corporate    
Statement [Line Items]    
Cost or Amortized Cost 892.4 1,036.5
Gross Unrealized Gains 2.7 14.0
Gross Unrealized Losses (20.2) (4.2)
Fair Market Value 874.9 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 229.8 196.1
Gross Unrealized Gains 2.4 6.9
Gross Unrealized Losses (4.7) (0.5)
Fair Market Value 227.5 202.5
Asset-backed    
Statement [Line Items]    
Cost or Amortized Cost 5.0 9.9
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.1) 0.0
Fair Market Value 4.9 9.9
Agency mortgage-backed    
Statement [Line Items]    
Cost or Amortized Cost 118.1 196.7
Gross Unrealized Gains 0.0 0.2
Gross Unrealized Losses (4.8) (1.4)
Fair Market Value 113.3 195.5
Total fixed income securities — Trading    
Statement [Line Items]    
Cost or Amortized Cost 1,434.2 1,634.9
Gross Unrealized Gains 5.1 21.5
Gross Unrealized Losses (32.1) (7.1)
Fair Market Value 1,407.2 1,649.3
Total short-term investments — Trading    
Statement [Line Items]    
Cost or Amortized Cost 4.3 73.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 4.3 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 37.7 33.5
Gross Unrealized Gains 0.1 0.0
Gross Unrealized Losses (0.4) (1.1)
Fair Market Value 37.4 32.4
Total    
Statement [Line Items]    
Cost or Amortized Cost 1,476.2 2,156.2
Gross Unrealized Gains 5.2 105.0
Gross Unrealized Losses (32.5) (15.5)
Fair Market Value $ 1,448.9 $ 2,245.7