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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 56.2 $ 363.8
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss [1] (0.6) (5.2)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 220.9 183.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss [1] (7.1) (4.1)
Corporate Debt Securities    
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 43.9 44.3
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss [1] (0.6) (0.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 103.4 100.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss [1] (3.7) (1.9)
Municipal Bonds    
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 12.3 64.5
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss [1] 0.0 (0.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 117.5 83.3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss [1] (3.4) (2.2)
U.S. Government Bonds    
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]    
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 0.0 255.0
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss [1] 0.0 (4.4)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 0.0 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss [1] $ 0.0 $ 0.0
[1] Represents the aggregate of the gross unrealized losses that have been in a continuous unrealized loss position as of December 31, 2018 and December 31, 2017.