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Significant Accounting Policies - Schedule of Balances of Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Valuation and Qualifying Accounts Disclosure [Line Items]    
Accounts, receivables deemed uncollectible, period 2 years  
Allowance for Doubtful Accounts    
Movement in Valuation Allowances and Reserves [Roll Forward]    
Balance at beginning of period $ 204,252 $ 196,270
Additions: charges to bad debt expense 22,153 16,502
Additions: charges to other accounts 0 503
Deductions (24,883) (17,571)
Balance at end of period $ 201,522 $ 195,704