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Available-for-Sale-Securities - Classification of Available-for-Sale Securities (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Available-for-sale Securities [Abstract]    
Cash and cash equivalents $ 12,378,000 $ 25,968,000
Marketable securities 27,597,303 25,916,681
Total $ 39,975,000 $ 51,885,000