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Fair Values of Financial Instruments - Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Balance at January 1 $ 3,074,227 $ 2,971,106
Sales (2,865,294)  
Principal payments received   (4,466)
Unrealized gains included in other comprehensive income (208,933) 107,587
Balance at December 31 $ 0 $ 3,074,227