Business Acquisitions (Details) - USD ($) $ in Thousands						1 Months Ended	12 Months Ended
	Nov. 27, 2017	Apr. 06, 2017	Dec. 15, 2016	Oct. 03, 2016	Sep. 23, 2016	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Acquisition related costs							$ 1,000	$ 5,500	$ 1,700
Allocation of the purchase consideration:
Goodwill							$ 1,487,404	$ 1,498,688	$ 1,228,503
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	1,082
Identifiable intangible assets	33,200
Goodwill	129,876
Fixed assets	1,570
Deferred tax assets	16,080
Other assets	19
Total assets acquired	200,127
Accounts payable	(1,501)
Accrued liabilities	(3,471)
Deferred revenue	(14,828)
Total liabilities assumed	(19,800)
Net assets acquired	180,327
Goodwill, expected tax deductible amount	54,100
Gross carrying amount of intangible assets	$ 33,200
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	$ 24,700
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.
Business Acquisition [Line Items]
Cash transferred		$ 199,300
Total purchase consideration		199,280
Allocation of the purchase consideration:
Cash		1,935
Accounts receivable		4,109
Prepaids and other current assets		4,384
Identifiable intangible assets		49,900
Goodwill		122,794
Deferred tax assets		31,206
Total assets acquired		214,328
Accounts payable		(1,119)
Accrued liabilities		(4,366)
Deferred revenue		(9,563)
Total liabilities assumed		(15,048)
Net assets acquired		199,280
Goodwill, expected tax deductible amount		36,400
Gross carrying amount of intangible assets		$ 49,900
Weighted average useful life		4 years 4 months 24 days
Soasta, Inc. | 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. | 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. | 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. | 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 6 days
Maximum potential payout of the contingent consideration			$ 10,500
Subsequent Event | Janrain, Inc.
Business Acquisition [Line Items]
Cash transferred						$ 125,000