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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($)
$ in Millions
Sep. 30, 2022
Dec. 31, 2021
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss    
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value $ 66.6 $ 151.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss (10.2) (3.8)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 562.2 259.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 44.4 3.7
Corporate debt securities    
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss    
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 39.0 3.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss (6.8) (0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 140.5 67.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 14.0 0.8
Municipal bonds    
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss    
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 26.8 4.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss (3.3) (0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 159.1 39.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 9.2 0.4
U.S. government bonds    
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss    
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 0.8 143.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss (0.1) (3.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 262.6 152.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss $ 21.2 $ 2.5