Income taxes - Net Operating Loss Carryforwards (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income taxes
Net operating loss carryforwards	$ 61,200,000
Valuation allowance	20,561,516	$ 28,475,080	$ 29,400,000
Valuation allowance deferred tax asset change in amount	6,200,000
Net Operating Loss Deferred Tax Asset	$ 16,600,000