Contract Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at Beginning of Period	$ 15,056
Charged to Cost and Expenses	21,263
Deductions	(23,954)
Balance at End of Period	12,365
Balance as of December 31, 2017	684,119
Recognition of unearned revenue	(1,860,169)
Deferral of revenue	1,896,745
Balance as of March 31, 2018	$ 684,119	$ 720,695
Remaining performance obligations	true