Summary of significant accounting policies - Allowance for Doubtful Accounts Receivable Rollforward (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts	$ 2,754,000	$ 2,740,000	$ 2,740,000
Allowance for Doubtful Accounts Receivable, Write-offs	(2,702,000)
Beginning Balance	2,754,000	2,740,000
Provision for Doubtful Accounts	23,000	14,000	13,000
Ending Balance	75,000	$ 2,754,000	$ 2,740,000
Natural Molecular Testing Corporation
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts	2,702,000
Ending Balance	$ 2,702,000