Summary of significant accounting policies - Stock based compensation expense and assumptions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock-based expense
Total stock-based compensation expense	$ 1,811,156	$ 1,263,660	$ 1,227,369
Capitalized stock-based compensation	$ 33,001	52,473	52,133
Increase (decrease) in expected stock price volatility	5.00%
Estimated change in stock based compensation expense	$ 25,000
Stock options
Stock-based expense
Total stock-based compensation expense	1,053,510	850,727	983,810
Capitalized stock-based compensation	100,000	100,000	100,000
Unamortized stock-based compensation expense	$ 1,354,926	$ 813,461
Weighted average remaining amortization period	1 year 8 months 23 days	1 year 10 months 6 days
Restricted stock
Stock-based expense
Total stock-based compensation expense	$ 757,646	$ 412,933	243,559
Unamortized stock-based compensation expense	$ 785,419	$ 424,795
Weighted average remaining amortization period	1 year 7 months 21 days	1 year 5 months 27 days
Processing and service costs
Stock-based expense
Total stock-based compensation expense	$ 179,750	$ 259,663	238,101
Selling general and administrative expenses
Stock-based expense
Total stock-based compensation expense	1,594,482	849,640	$ 989,268
Restructuring charge
Stock-based expense
Total stock-based compensation expense	$ 36,924	$ 154,357