Annual report pursuant to Section 13 and 15(d)

Organization and summary of significant accounting policies (Schedule of Trading Securities) (Details)

v3.7.0.1
Organization and summary of significant accounting policies (Schedule of Trading Securities) (Details) - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Trading securities cost, current $ 7,516,404 $ 14,174,502
Trading securities fair value, current 7,506,761 14,147,991
Total trading securities cost 7,516,404 15,151,124
Total trading securities, fair value 7,506,761 15,119,991
Certificates of deposit / commercial paper [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Trading securities cost, current 2,378,222 1,249,988
Trading securities fair value, current 2,373,891 1,248,845
Corporate Bonds [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Trading securities cost, current 5,138,182 12,924,514
Trading securities fair value, current 5,132,870 12,899,146
Trading securities cost, noncurrent 626,622
Trading securities fair value, noncurrent 622,987
Certificates of deposit [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Trading securities cost, noncurrent 350,000
Trading securities fair value, noncurrent $ 349,013