Fair value disclosures - Liabilities measured at fair value on recurring basis (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Penalty payment derivative	$ 400
Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Penalty payment derivative	389	$ 389
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Penalty payment derivative	$ 389	$ 389