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Summary of Significant Accounting Policies (Allowance for doubtful accounts) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at beginning of period $ 5,795 $ 3,331 $ 8,119
Charged to expense 3,342 2,903 485
Deductions or other (1,705) (439) (5,273)
Balance at end of period $ 7,432 $ 5,795 $ 3,331