Nature of operations and summary of significant accounting policies (Income Taxes) (Details) - USD ($)	3 Months Ended	6 Months Ended	9 Months Ended
	Mar. 21, 2017	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Deferred tax benefit		$ 0
Uncertain tax positions		$ 0	$ 0
Estimated amount of cancellation of indebtedness income			$ 61,000,000
Predecessor
Income Tax Disclosure [Line Items]
Deferred tax benefit	$ 0
Uncertain tax positions				$ 0