Contract Balances (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	$ 15,056	$ 17,440	$ 21,769
Charged to Cost and Expenses	98,461	124,143	149,599
Deductions	(96,561)	(126,527)	(153,928)
Balance at End of Period	16,956	15,056	$ 17,440
Balance at Beginning of Period	684,119
Recognition of unearned revenue	(6,850,394)
Deferral of revenue	6,790,901
Balance at End of Period	$ 624,626	$ 684,119
Remaining performance obligations	true