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Compensation Plans - 401(k) Plan (Details) - 401K - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Issuance of common stock related to 401k plan match (in shares) 8,478 9,466 9,336
Expenses attributable to the 401(k) Plan $ 1,710,785 $ 1,738,287 $ 1,645,434