Goodwill and Acquired Intangible Assets - Schedule of the Changes in the Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Changes in the carrying amount of goodwill
Beginning balance	$ 1,228,503	$ 1,150,244
Measurement period adjustments	4,217	0
Foreign currency translation	10,546	(5,089)
Ending balance	1,498,688	1,228,503
Concord Systems, Inc.
Changes in the carrying amount of goodwill
Acquisitions	0	1,079
Soha Systems, Inc.
Changes in the carrying amount of goodwill
Acquisitions	0	43,515
Cyberfend, Inc.
Changes in the carrying amount of goodwill
Acquisitions	0	38,754
Soasta, Inc. [Member]
Changes in the carrying amount of goodwill
Acquisitions	121,668	0
Nominum, Inc.
Changes in the carrying amount of goodwill
Acquisitions	$ 133,754	$ 0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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