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Equity Based Plans, Summary of Restricted Stock Cancelled Due to Employee Terminations Prior to Restrictions Lapsing (Details) - Restricted Stock [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares (in shares) 2,875 4,965 0
Weighted average fair value (in dollars per share) $ 63.12 $ 35.78 $ 0