Nature of operations and summary of significant accounting policies (Income Taxes) (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Accounting Policies [Abstract]
Effective tax rate	0.00%	0.00%	0.00%	0.00%
Deferred tax benefit	$ 0		$ 0
Uncertain tax positions	0		0		$ 0
Net operating loss carryforwards subject to limitation	$ 760,067,000		$ 760,067,000

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
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-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration