Basic and diluted earnings (loss) per share - Computation of our diluted earnings (loss) 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
Computation of our diluted (loss) earnings per share
Anti-dilutive instruments	31.7	10.7	5.4	10.7
Share-based compensation.
Computation of our diluted (loss) earnings per share
Anti-dilutive instruments	2.6	2.6	5.4	2.6
Convertible Debentures
Computation of our diluted (loss) earnings per share
Anti-dilutive instruments	29.1	8.1		8.1

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration