Summary of significant accounting policies - Stock based compensation expense and assumptions (Details) - USD ($)	12 Months Ended			36 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Stock-based expense
Total stock-based compensation expense	$ 1,820,404	$ 1,811,156	$ 1,263,660
Capitalized stock-based compensation	$ 27,703	33,001	52,473
Increase (decrease) in expected stock price volatility	5.00%
Estimated change in stock based compensation expense	$ 10,000
Stock options
Stock-based expense
Total stock-based compensation expense	988,890	1,053,510	850,727
Capitalized stock-based compensation				$ 100,000
Unamortized stock-based compensation expense	$ 956,947	$ 1,354,926		956,947
Weighted average remaining amortization period	1 year 6 months 26 days	1 year 8 months 23 days
Restricted stock
Stock-based expense
Total stock-based compensation expense	$ 831,514	$ 757,646	412,933
Unamortized stock-based compensation expense	$ 206,963	$ 785,419		$ 206,963
Weighted average remaining amortization period	7 months 6 days	1 year 7 months 21 days
Processing and service costs
Stock-based expense
Total stock-based compensation expense	$ 174,027	$ 179,750	259,663
Selling general and administrative expenses
Stock-based expense
Total stock-based compensation expense	$ 1,646,377	1,594,482	849,640
Restructuring charge
Stock-based expense
Total stock-based compensation expense		$ 36,924	$ 154,357