Summary of business and significant accounting policies (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Allowance for Doubtful Accounts Receivable, Current		$ 0.8	$ 1.3
Amortization of Long-term Assets		$ 19.2	19.6	$ 16.8
Warranty Period		12 months
Advertising Expense		$ 61.3	106.0	$ 64.7
Effective tax rate reconciliation, share based compensation, excess tax benefit			179.0
Minimum [Member]
Property, Plant and Equipment [Line Items]
Long-term Assets, Amortization Period		24 months
Useful Life		1 year
Maximum [Member]
Property, Plant and Equipment [Line Items]
Long-term Assets, Amortization Period		36 months
Useful Life		11 years
Accounting Standards Update 2014-09, 2016-08, 2016-10, 2016-12 [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Sales incentives	$ 5.0
New Accounting Pronouncement, Early Adoption, Effect [Member] | Accounting Standards Update 2016-09 [Member]
Property, Plant and Equipment [Line Items]
Deferred tax liability			16.0
Deferred income tax assets			163.0
New Accounting Pronouncement, Early Adoption, Effect [Member] | Retained Earnings [Member] | Accounting Standards Update 2016-09 [Member]
Property, Plant and Equipment [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ 16.0
European Union [Member]
Property, Plant and Equipment [Line Items]
Warranty Period		2 years