Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue Recognition
Sales return period prior to expiry date	6 months
Sales return period subsequent to expiry date	12 months
Milestone revenues recorded	$ 1.5	$ 0.0	$ 0.8
Selling, general and administrative
Advertising Expense
Advertising costs	$ 33.8	$ 21.9	$ 19.3