Net income (loss) per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income (loss)	$ 92.7	$ (9.5)	$ 80.2	$ (35.6)
Denominator:
Weighted-average number of shares-basic (in shares)	52.6	51.5	52.3	51.3
Dilutive securities-equity awards (in shares)	0.9	0.0	0.9	0.0
Weighted-average number of shares-diluted (in shares)	53.5	51.5	53.2	51.3
Net income (loss) per share - basic (in dollars per share)	$ 1.76	$ (0.18)	$ 1.53	$ (0.69)
Net income (loss) per share - diluted (in dollars per share)	$ 1.73	$ (0.18)	$ 1.51	$ (0.69)
Antidilutive securities excluded from computation of earnings per share (in shares)	0.4	3.0	0.6	3.0