Business Acquisitions (Details) - USD ($) $ in Thousands									12 Months Ended
	Nov. 27, 2017	Apr. 07, 2017	Dec. 15, 2016	Oct. 03, 2016	Sep. 23, 2016	Oct. 30, 2015	Apr. 06, 2015	Feb. 27, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 03, 2016
Business Acquisition [Line Items]
Acquisition related costs									$ 5,500	$ 1,700	$ 1,800
Allocation of the purchase consideration:
Goodwill									$ 1,498,688	$ 1,228,503	$ 1,150,244
Nominum, Inc.
Business Acquisition [Line Items]
Cash transferred	$ 180,300
Total purchase consideration	180,327
Allocation of the purchase consideration:
Cash	8,455
Accounts receivable	9,845
Prepaids and other current assets	1,481
Identifiable intangible assets	32,800
Goodwill	133,754
Fixed assets	2,169
Deferred tax assets	11,398
Other assets	19
Total assets acquired	199,921
Accounts payable	(1,460)
Accrued liabilities	(3,306)
Deferred revenue	(14,828)
Total liabilities assumed	(19,594)
Net assets acquired	180,327
Goodwill, expected tax deductible amount	44,200
Gross carrying amount of intangible assets	$ 32,800
Weighted average useful life	5 years 4 months 24 days
Nominum, Inc. | Completed technologies
Allocation of the purchase consideration:
Gross carrying amount of intangible assets	$ 7,200
Weighted average useful life	2 years 2 months 12 days
Nominum, Inc. | Customer-related intangible assets
Allocation of the purchase consideration:
Gross carrying amount of intangible assets	$ 24,300
Weighted average useful life	6 years 6 months
Nominum, Inc. | Trademarks
Allocation of the purchase consideration:
Gross carrying amount of intangible assets	$ 1,100
Weighted average useful life	3 years 8 months 12 days
Nominum, Inc. | Non-compete agreements
Allocation of the purchase consideration:
Gross carrying amount of intangible assets	$ 200
Weighted average useful life	1 year 6 months
Soasta, Inc. [Member]
Business Acquisition [Line Items]
Cash transferred		$ 199,300
Total purchase consideration		199,280
Allocation of the purchase consideration:
Cash		1,935
Accounts receivable		4,108
Prepaids and other current assets		1,143
Identifiable intangible assets		49,900
Goodwill		125,584
Deferred tax assets		31,206
Total assets acquired		213,876
Accounts payable		(1,119)
Accrued liabilities		(3,915)
Deferred revenue		(9,562)
Total liabilities assumed		(14,596)
Net assets acquired		199,280
Goodwill, expected tax deductible amount		35,600
Gross carrying amount of intangible assets		$ 49,900
Weighted average useful life		4 years 4 months 24 days
Soasta, Inc. [Member] | Completed technologies
Allocation of the purchase consideration:
Gross carrying amount of intangible assets		$ 18,800
Weighted average useful life		4 years 1 month 6 days
Soasta, Inc. [Member] | Customer-related intangible assets
Allocation of the purchase consideration:
Gross carrying amount of intangible assets		$ 28,200
Weighted average useful life		4 years 7 months 6 days
Soasta, Inc. [Member] | Trademarks
Allocation of the purchase consideration:
Gross carrying amount of intangible assets		$ 2,400
Weighted average useful life		4 years 10 months 24 days
Soasta, Inc. [Member] | Non-compete agreements
Allocation of the purchase consideration:
Gross carrying amount of intangible assets		$ 500
Weighted average useful life		1 year 10 months 24 days
Concord Systems, Inc.
Business Acquisition [Line Items]
Cash transferred					$ 3,000
Allocation of the purchase consideration:
Identifiable intangible assets					2,800
Goodwill					$ 1,100
Weighted average useful life					7 years
Soha Systems, Inc.
Business Acquisition [Line Items]
Total purchase consideration				$ 55,000
Allocation of the purchase consideration:
Identifiable intangible assets				10,700
Goodwill				44,100
Goodwill, expected tax deductible amount				$ 12,000
Weighted average useful life				4 years 8 months 12 days
Maximum potential payout of the contingent consideration												$ 5,000
Cyberfend, Inc.
Business Acquisition [Line Items]
Total purchase consideration			$ 37,500
Allocation of the purchase consideration:
Identifiable intangible assets			6,500
Goodwill			38,500
Goodwill, expected tax deductible amount			$ 11,000
Weighted average useful life			3 years 7 months
Maximum potential payout of the contingent consideration			$ 10,500
Xerocole, Inc.
Business Acquisition [Line Items]
Total purchase consideration								$ 16,600
Allocation of the purchase consideration:
Identifiable intangible assets								4,900
Goodwill								12,900
Goodwill, expected tax deductible amount								$ 2,700
Weighted average useful life								8 years 9 months 18 days
Octoshape
Business Acquisition [Line Items]
Cash transferred							$ 107,000
Total purchase consideration							107,047
Allocation of the purchase consideration:
Cash							664
Accounts receivable							1,976
Other current assets							393
Identifiable intangible assets							41,950
Goodwill							69,445
Deferred tax assets							5,230
Total assets acquired							119,658
Other current liabilities							(1,983)
Deferred revenue							(770)
Deferred tax liabilities							(9,858)
Total liabilities assumed							(12,611)
Net assets acquired							107,047
Goodwill, expected tax deductible amount							69,400
Gross carrying amount of intangible assets							$ 41,950
Weighted average useful life							10 years 7 months 6 days
Octoshape | Completed technologies
Allocation of the purchase consideration:
Gross carrying amount of intangible assets							$ 25,310
Weighted average useful life							9 years 9 months
Octoshape | Customer-related intangible assets
Allocation of the purchase consideration:
Gross carrying amount of intangible assets							$ 16,560
Weighted average useful life							11 years 9 months
Octoshape | Non-compete agreements
Allocation of the purchase consideration:
Gross carrying amount of intangible assets							$ 80
Weighted average useful life							2 years
Bloxx
Business Acquisition [Line Items]
Total purchase consideration						$ 18,700
Allocation of the purchase consideration:
Identifiable intangible assets						3,900
Goodwill						17,700
Goodwill, expected tax deductible amount						$ 17,700
Weighted average useful life						7 years 2 months 12 days