Revenues - Impact of Adoption of ASC 606 on Company's Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
ASSETS
Accounts receivable, net of allowance for doubtful accounts of $7,390 and $6,739, respectively	$ 264,076	$ 243,900
Contract asset (current)	11,942
Prepaid expenses and other current assets	35,559	38,742
Contract asset (non-current)	22,076
Liabilities:
Deferred revenue	193,024	204,601
Other long-term liabilities	120,621	$ 118,679
ASU 2014-09 [Member] | Balance Without Adoption of ASC 606 [Member]
ASSETS
Accounts receivable, net of allowance for doubtful accounts of $7,390 and $6,739, respectively	265,272
Prepaid expenses and other current assets	31,943
Liabilities:
Deferred revenue	230,504
Other long-term liabilities	118,115
ASU 2014-09 [Member] | Effect of Change [Member]
ASSETS
Accounts receivable, net of allowance for doubtful accounts of $7,390 and $6,739, respectively	(1,196)
Contract asset (current)	11,942
Prepaid expenses and other current assets	3,616
Contract asset (non-current)	22,076
Liabilities:
Deferred revenue	(37,480)
Other long-term liabilities	$ 2,506