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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
Mar. 31, 2018
Dec. 31, 2017
Statement [Line Items]    
Fair Market Value $ 1,637.6 $ 1,649.3
U.S. government    
Statement [Line Items]    
Cost or Amortized Cost 141.8 162.3
Gross Unrealized Gains 0.6 0.4
Gross Unrealized Losses (0.7) (0.8)
Fair Market Value 141.7 161.9
Municipal    
Statement [Line Items]    
Cost or Amortized Cost 75.0 32.4
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.7) (0.2)
Fair Market Value 74.3 32.2
Corporate    
Statement [Line Items]    
Cost or Amortized Cost 1,014.5 1,036.5
Gross Unrealized Gains 5.8 14.0
Gross Unrealized Losses (16.7) (4.2)
Fair Market Value 1,003.6 1,046.3
Non-U.S. government-backed corporate    
Statement [Line Items]    
Cost or Amortized Cost 1.0 1.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 1.0 1.0
Non-U.S. government    
Statement [Line Items]    
Cost or Amortized Cost 216.2 196.1
Gross Unrealized Gains 3.4 6.9
Gross Unrealized Losses (3.0) (0.5)
Fair Market Value 216.6 202.5
Asset-backed    
Statement [Line Items]    
Cost or Amortized Cost 8.4 9.9
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.1) 0.0
Fair Market Value 8.3 9.9
Agency mortgage-backed    
Statement [Line Items]    
Cost or Amortized Cost 195.8 196.7
Gross Unrealized Gains 0.3 0.2
Gross Unrealized Losses (4.0) (1.4)
Fair Market Value 192.1 195.5
Total fixed income securities — Trading    
Statement [Line Items]    
Cost or Amortized Cost 1,652.7 1,634.9
Gross Unrealized Gains 10.1 21.5
Gross Unrealized Losses (25.2) (7.1)
Fair Market Value 1,637.6 1,649.3
Total short-term investments — Trading    
Statement [Line Items]    
Cost or Amortized Cost 27.0 73.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 27.0 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 35.0 33.5
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.2) (1.1)
Fair Market Value 34.8 32.4
Total    
Statement [Line Items]    
Cost or Amortized Cost 1,714.7 2,156.2
Gross Unrealized Gains 10.1 105.0
Gross Unrealized Losses (25.4) (15.5)
Fair Market Value $ 1,699.4 $ 2,245.7