Supplemental Information (Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of expenses in statements of operations
Research and development	$ 73.6	$ 64.6	$ 62.3
Advertising, promotions and marketing	45.0	41.0	42.5
Cooperative advertising expenditures	18.6	14.7	13.7
Rent expense	$ 57.7	$ 57.9	$ 53.5