Cash and Marketable Securities - Tables 4 & 5 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Summary of Restricted Marketable Securities
Restricted marketable securities, cost	$ 58.3	$ 51.3
Restricted marketable securities, gross unrealized gains	0.3	0.5
Restricted marketable securities, gross unrealized losses	(2.4)	(1.3)
Restricted marketable securities, fair value	$ 56.2	$ 50.5