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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, 2017
Dec. 31, 2016
Statement [Line Items]    
Fair Market Value $ 1,357.5 $ 1,265.7
U.S. government    
Statement [Line Items]    
Cost or Amortized Cost 112.4 82.8
Gross Unrealized Gains 0.7 0.4
Gross Unrealized Losses (0.3) (0.8)
Fair Market Value 112.8 82.4
Municipal    
Statement [Line Items]    
Cost or Amortized Cost 5.7 15.7
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 (0.2)
Fair Market Value 5.7 15.5
Corporate    
Statement [Line Items]    
Cost or Amortized Cost 882.5 817.8
Gross Unrealized Gains 16.1 9.9
Gross Unrealized Losses (2.7) (7.1)
Fair Market Value 895.9 820.6
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 180.8 203.4
Gross Unrealized Gains 6.3 3.5
Gross Unrealized Losses (0.6) (4.1)
Fair Market Value 186.5 202.8
Asset-backed    
Statement [Line Items]    
Cost or Amortized Cost 12.2 14.5
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 12.2 14.5
Agency mortgage-backed    
Statement [Line Items]    
Cost or Amortized Cost 142.7 130.6
Gross Unrealized Gains 0.4 0.2
Gross Unrealized Losses (0.7) (0.9)
Fair Market Value 142.4 129.9
Total fixed income securities — Trading    
Statement [Line Items]    
Cost or Amortized Cost 1,338.3 1,264.8
Gross Unrealized Gains 23.5 14.0
Gross Unrealized Losses (4.3) (13.1)
Fair Market Value 1,357.5 1,265.7
Total short-term investments — Trading    
Statement [Line Items]    
Cost or Amortized Cost 73.7 185.4
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 73.7 185.4
Total equity securities — Trading    
Statement [Line Items]    
Cost or Amortized Cost 569.5 554.3
Gross Unrealized Gains 99.4 55.4
Gross Unrealized Losses (10.2) (25.0)
Fair Market Value 658.7 584.7
Total catastrophe bonds — Trading    
Statement [Line Items]    
Cost or Amortized Cost 28.3 42.5
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 28.3 42.5
Total    
Statement [Line Items]    
Cost or Amortized Cost 2,009.8 2,047.0
Gross Unrealized Gains 122.9 69.4
Gross Unrealized Losses (14.5) (38.1)
Fair Market Value $ 2,118.2 $ 2,078.3