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Summary of Significant Accounting Policies - Cash and Cash Equivalents (Narrative) (Details) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Summary of Significant Accounting Policies        
Cash and cash equivalents $ 4,131,000 $ 1,856,000 $ 1,891,000 $ 26,780,000