Summary of Significant Accounting Policies - Sales and Marketing Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Marketing and advertising expenses	$ 71.0	$ 70.7	$ 79.6
Prepaid media costs	0.4	1.6
Accrued marketing and advertising expenses	$ 2.7	$ 2.1