Income taxes - Additional Information (Details) $ in Millions	Jun. 30, 2017 USD ($)
Operating loss carry-forward
Deferred tax assets, valuation allowance	$ 716.2
Internal Revenue Service (IRS) \| Federal
Operating loss carry-forward
Operating loss carryforwards	1,700.0
State of Oklahoma \| State
Operating loss carry-forward
Operating loss carryforwards	$ 42.5

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32537-109319

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32559-109319

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