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Significant Accounting Policies - Schedule of Balances of Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]    
Accounts, receivables deemed uncollectible, period 2 years  
Allowance for Doubtful Accounts    
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]    
Balance at beginning of period $ 204,252 $ 196,270
Additions: charges to bad debt expense 58,282 51,439
Additions: charges to other accounts 1,124 190
Deductions (65,635) (45,490)
Balance at end of period $ 198,023 $ 202,409