Income taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Provision for income taxes	$ 0.0	$ 0.0
Operating Loss Carryforwards	$ 396.6
Operating loss carryforwards, expiration date	Dec. 31, 2021
Annual limit on operating loss carryforwards	In certain circumstances, as specified in the Internal Revenue Code, a 50% or more ownership change by certain combinations of our shareholders during any three year period would result in limitations on our ability to use a portion of our net operating loss carryforwards.
Accrued penalties and interest	$ 0.0
Unrecognized tax benefits	$ 0.0	$ 0.0
Tax years open for examination	1998
Tax Credit Carryforward, Description	At December 31, 2020, we have net operating loss carryforwards of approximately $396.6 million for federal income tax reporting purposes. In addition, we have research and development tax credits of $8.8 million. During 2020, $28.4 million federal net operating losses and $512,000 general business credits 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 2021 to 2040, if not previously used.
Capital Loss Carryforward
Tax Credit Carryforwards Expired during year	$ 28.4
R&D Tax Credit
Tax Credit Carryforward, Amount	$ 8.8
Tax Credit Carryforward, Expiration Date	Dec. 31, 2021
Tax Credit Carryforwards Expired during year	$ 0.5