| Basic and diluted net loss per share | 
    |  |  | Three Months Ended March 31, |  |  | Six Months Ended March 31, |  |  
    |  |  | 2018 |  |  | 2017 |  |  | 2018 |  |  | 2017 |  |  
    | Basic |  |  |  |  |  |  |  |  |  |  |  |  |  
    |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    | Net income |  | $ | 1,923,273 |  |  | $ | 1,840,666 |  |  | $ | 3,800,828 |  |  | $ | 3,269,239 |  |  
    | Less: preferred stock dividends |  |  | (292 | ) |  |  | (480 | ) |  |  | (584 | ) |  |  | (959 | ) |  
    | Net income applicable to common stock |  | $ | 1,922,981 |  |  | $ | 1,840,186 |  |  | $ | 3,800,244 |  |  | $ | 3,268,280 |  |  
    |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    | Weighted average common shares outstanding |  |  | 1,970,136 |  |  |  | 2,058,064 |  |  |  | 1,972,758 |  |  |  | 2,029,023 |  |  
    |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    | Basic earnings per share |  | $ | 0.98 |  |  | $ | 0.89 |  |  | $ | 1.93 |  |  | $ | 1.61 |  |     
    | Diluted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    | Net income applicable to common stock |  | $ | 1,922,981 |  |  | $ | 1,840,186 |  |  | $ | 3,800,244 |  |  | $ | 3,268,280 |  |  
    | Add: preferred stock dividends |  |  | 292 |  |  |  | 480 |  |  |  | 292 |  |  |  | 959 |  |  
    | Net income applicable for diluted earnings per share |  | $ | 1,923,273 |  |  | $ | 1,840,666 |  |  | $ | 3,800,536 |  |  | $ | 3,269,239 |  |  
    |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    | Weighted average common shares outstanding |  |  | 2,027,564 |  |  |  | 2,058,064 |  |  |  | 1,972,758 |  |  |  | 2,029,023 |  |  
    | Add: Options |  |  | 44,923 |  |  |  | 69,979 |  |  |  | 44,171 |  |  |  | 56,458 |  |  
    | Add: Common Stock Warrants |  |  | – |  |  |  | – |  |  |  | – |  |  |  | – |  |  
    | Add: Series B Preferred Stock |  |  | 1,071,200 |  |  |  | 1,071,200 |  |  |  | 1,071,200 |  |  |  | 1,071,200 |  |  
    | Add: Series B Preferred Stock Warrants |  |  | 590,145 |  |  |  | 590,145 |  |  |  | 590,145 |  |  |  | 590,145 |  |  
    | Add: Series E Preferred Stock |  |  | 77,840 |  |  |  | 127,840 |  |  |  | 77,840 |  |  |  | 127,840 |  |  
    | Assumed weighted average common shares outstanding |  |  | 3,811,672 |  |  |  | 3,917,228 |  |  |  | 3,756,114 |  |  |  | 3,874,666 |  |  
    |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    | Diluted earnings per share |  | $ | 0.50 |  |  | $ | 0.47 |  |  | $ | 1.01 |  |  | $ | 0.84 |  |    |