Fair value disclosures - Assets and liabilities measured at fair value on recurring basis (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Measurements, Recurring | Rusnano payment derivative
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	$ 0	$ 2,000,000
Fair Value, Measurements, Recurring | Rusnano payment derivative | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	0
Fair Value, Measurements, Recurring | Rusnano payment derivative | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	0
Fair Value, Measurements, Recurring | Rusnano payment derivative | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	2,000,000
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	7,567,000	7,481,000
Fair Value, Measurements, Recurring | Money market funds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	7,567,000	7,481,000
Fair Value, Measurements, Recurring | Money market funds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	0
Fair Value, Measurements, Recurring | Money market funds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	0
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	560,000	465,000
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	560,000	465,000
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	0
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	0	0
Fair Value, Measurements, Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net asset (liability)	$ 0	$ 0