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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