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Significant Accounting Policies - Doubtful Accounts (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Schedule of allowance for doubtful accounts      
Beginning allowance for doubtful accounts $ 10,024 $ 8,835 $ 10,303
Additions charged to expense 16,503 13,067 16,947
Write-offs, net of recoveries (16,326) (11,878) (18,415)
Ending allowance for doubtful accounts $ 10,201 $ 10,024 $ 8,835