Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2017

Investments Schedule [Abstract]

Schedule of Marketable Securities

The following table presents the values of our marketable securities as of the dates shown.

	September 30, 2017				December 31, 2016
(in millions)	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Zero coupon bonds	$	3.8	$	3.8	$	3.8	$	3.8
U.S. treasury and government agencies	6.0		6.0		8.0		8.1
Asset-backed securities	0.3		0.3		0.4		0.4
Corporate debt securities	11.2		11.4		14.4		14.5
State and political subdivisions	22.7		22.5		26.2		25.7
Total marketable securities	$	44.0	$	44.0	$	52.8	$	52.5