Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale securities, less than twelve months, fair value	$ 40.0	$ 225.0
Available-for-sale securities, less than twelve months, gross unrealized losses	(0.7)	(2.6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5.4	35.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(0.1)	(0.4)
Corporate debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale securities, less than twelve months, fair value	9.2	17.9
Available-for-sale securities, less than twelve months, gross unrealized losses	(0.2)	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	20.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	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	25.6	11.7
Available-for-sale securities, less than twelve months, gross unrealized losses	0.0	(0.1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5.4	14.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(0.1)	(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	5.2	195.4
Available-for-sale securities, less than twelve months, gross unrealized losses	(0.5)	(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	$ 0.0	$ 0.0