Investments - Available-for-sale Securities (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014	Dec. 31, 2014
Available-for-sale Securities [Abstract]
Cost	$ 20.1			$ 20.1			$ 19.6
Gross Unrealized Gain	0.1			0.1			0.2
Gross Unrealized Loss	1.7			1.7			0.9
Fair Value	18.5	[1]		18.5	[1]		$ 18.9
Net Proceeds	0.0		$ 0.6	0.7		$ 3.3
Gross Realized Gain	0.0		0.0	0.1		0.2
Gross Realized Loss	$ 0.0		$ 0.0	$ 0.0		$ 0.0

[1] As of September 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 $1.6 million, in three years to less than five years was $3.0 million, and in five or more years was $6.4 million.