Investments - Available-for-sale Securities (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Available-for-sale Securities [Abstract]
Cost		$ 19.8		$ 19.8		$ 19.6
Gross Unrealized Gain		0.3		0.3		0.2
Gross Unrealized Loss		0.8		0.8		0.9
Fair Value	[1]	19.3		19.3		$ 18.9
Net Proceeds		0.5	$ 2.1	0.7	$ 2.7
Gross Realized Gain		0.1	0.2	0.1	0.2
Gross Realized Loss		$ 0.0	$ 0.0	$ 0.0	$ 0.0

[1] As of June 30, 2015, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $1.7 million, in three years to less than five years was $3.1 million, and in five or more years was $6.2 million.