Significant Accounting Policies - Schedule of Balances of Allowance for Doubtful Accounts (Details) - Allowance for Doubtful Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 196,270	$ 161,658	$ 170,140
Additions: charges to bad debt expense	124,308	108,019	107,162
Additions: charges to other accounts	0	6,908	0
Deductions	(116,326)	(80,315)	(115,644)
Balance at end of period	$ 204,252	$ 196,270	$ 161,658