Loss per share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic and diluted:
Net loss attributable to common shareholders	$ (68)	$ (33)	$ (162)	$ (81)
Denominator:
Basic: Weighted average number of shares outstanding (in millions) (in shares)	87,150,000	87,130,000	87,150,000	87,130,000
Diluted: Weighted-average number of shares outstanding, assuming dilution (in millions) (in shares)	87,150,000	87,130,000	87,150,000	87,130,000
Basic loss per share (in dollars per share)	$ (0.78)	$ (0.38)	$ (1.86)	$ (0.93)
Diluted loss per share (in dollars per share)	$ (0.78)	$ (0.38)	$ (1.86)	$ (0.93)
Shares excluded from calculation (in shares)	815,627	815,627	815,627	815,627