Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Realized Losses	$ (3.5)	$ (10.5)
Realized Gains	4.7	$ 0.2
Debt Securities [Member]
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in one year or less	28.4
Due after one year through five years	367.8
Due after five years through ten years	181.0
Due after ten years	46.7
Total debt securities	$ 623.9