Earnings (loss) per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income (loss)	$ (20.3)	$ 20.8	$ (1.2)	$ 12.8
Amount allocated to common stockholders (as percent)	100.00%	92.06%	100.00%	92.04%
Basic EPS:
Net income (loss) allocated to common stockholders	$ (20.3)	$ 19.1	$ (1.2)	$ 11.8
Net income (loss) allocated to common stockholders (shares)	87.6	87.3	87.6	87.3
Net income (loss) allocated to common stockholders, per share (in USD per share)	$ (0.23)	$ 0.22	$ (0.01)	$ 0.13
Effect of Dilutive Securities:
Share-based compensation	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Share-based compensation (shares)	0.0	0.9	0.0	0.8
Diluted EPS:
Net income (loss) allocated to common stockholders with assumed conversion	$ (20.3)	$ 19.1	$ (1.2)	$ 11.8
Net income (loss) allocated to common stockholders with assumed conversion (shares)	87.6	88.3	87.6	88.1
Net income (loss) allocated to common stockholders with assumed conversion, per share (in USD per share)	$ (0.23)	$ 0.22	$ (0.01)	$ 0.13
Securities excluded from calculation of diluted EPS (shares)	1.3	0.5	1.4	0.5