Investments - Available-for-sale Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Available-for-sale Securities [Abstract]
Cost	$ 20.0		$ 19.6	$ 18.3
Gross Unrealized Gain	0.0		0.2	0.0
Gross Unrealized Loss	1.5		0.9	0.6
Fair Value	18.5	[1]	18.9	17.7
Net Proceeds	1.7		3.6	16.1
Gross Realized Gain	0.1		0.2	2.2
Gross Realized Loss	$ 0.0		$ 0.0	$ 0.0

[1] As of December 31, 2015, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was none, in one year to less than three years was $1.0 million, in three years to less than five years was $3.2 million, and in five or more years was $6.7 million.