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Earnings (loss) per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income (loss)	$ 19.4		$ 7.1		$ 19.1	$ (8.0)
Amount allocated to common stockholders (as percent)	92.20%		92.00%		92.20%	100.00%
Basic EPS:
Net income allocated to common stockholders	$ 17.9		$ 6.6		$ 17.6	$ (8.0)
Net loss allocated to common stockholders (shares)	87.6		87.3		87.6	87.3
Net loss allocated to common stockholders, per share (in USD per share)	$ 0.20		$ 0.08		$ 0.20	$ (0.09)
Effect of Dilutive Securities:
Share-based compensation	$ 0.0		$ 0.0		$ 0.0	$ 0.0
Share-based compensation (shares)	0.8		0.8		0.8	0.0
Diluted EPS:
Net income allocated to common stockholders with assumed conversion	$ 17.9		$ 6.6		$ 17.6	$ (8.0)
Net loss allocated to common stockholders with assumed conversion (shares)	88.4		88.1		88.4	87.3
Net loss allocated to common stockholders with assumed conversion, per share (in USD per share)	$ 0.20		$ 0.07		$ 0.20	$ (0.09)
Securities excluded from calculation of diluted EPS (shares)		0.4		0.6	0.6	0.5

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