Revenue Recognition - Summary of Deferred Revenue Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Deferred Revenue [Roll Forward]
Deferred revenue balance, beginning of period	$ 67.4	$ 57.8	$ 59.0
New deferrals	205.6	200.6	252.0
Amounts recognized in revenue	(217.6)	(191.0)	(253.2)
Deferred revenue balance, end of period	55.4	$ 67.4	$ 57.8
Revenue recognized, previously deferred	$ 57.3