Shares used to calculate earnings per share (Details) - shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Basic (shares)	51.2	50.7	51.0	50.6
Effect of dilutive stock options and awards (shares)	0.0	0.9	0.0	0.9
Diluted (shares)	51.2	51.6	51.0	51.5
Antidilutive stock options and awards excluded from denominator (shares)	1.6	0.3	1.7	0.2
Share-based goods and nonemployee services transaction, quantity of securities issued (shares)	0.3	0.3	0.3	0.3