Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale securities, less than twelve months, fair value		$ 126.1	$ 225.0
Available-for-sale securities, less than twelve months, gross unrealized losses	[1]	(3.4)	(2.6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		8.8	35.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[1]	(0.3)	(0.4)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		134.9	260.4
Debt Securities, Available-for-sale, Unrealized Loss Position		(3.7)	(3.0)
Corporate debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale securities, less than twelve months, fair value		68.1	17.9
Available-for-sale securities, less than twelve months, gross unrealized losses	[1]	(2.5)	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		3.0	20.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[1]	0.0	(0.1)
Municipal bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale securities, less than twelve months, fair value		57.9	11.7
Available-for-sale securities, less than twelve months, gross unrealized losses	[1]	(0.9)	(0.1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		5.8	14.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[1]	(0.3)	(0.3)
U.S. government bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale securities, less than twelve months, fair value		0.1	195.4
Available-for-sale securities, less than twelve months, gross unrealized losses	[1]	0.0	(2.5)
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

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