Income taxes (Credit Carryforwards Narrative) (Details) - R&D Tax Credit $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Tax Credit Carryforward, Description	<p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0">At December 31, 2018, we have net operating loss carryforwards of approximately $398.1 million for federal income tax reporting purposes. In addition, we have research and development tax credits of $8.6 million. During 2018, $7.1 million federal net operating losses and $310,000 capital losses expired unused. A majority of the net operating loss carryforwards and research and development credits available to offset future taxable income, if any, will expire in varying amounts from 2019 to 2038, if not previously used.</p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"></p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"></p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"></p>
Tax Credit Carryforward, Amount	$ 8.6
Tax Credit Carryforward, Expiration Date	Dec. 31, 2019