Income taxes - Additional Information (Details) $ in Millions	Jun. 30, 2018 USD ($)
Operating loss carry-forward
Net operating loss carry-forward that will not expire due to TCJA	$ 23.0
Deferred tax assets, valuation allowance	315.1
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	$ 40.3

Tax Cuts And Jobs Act Of 2017, Net Operating Loss Carry-forward That Will Not Expire

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

+ References

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