Business Acquisitions and Divestitures (Details) - USD ($) $ in Thousands								3 Months Ended		12 Months Ended
	Dec. 15, 2016	Oct. 03, 2016	Sep. 23, 2016	Oct. 30, 2015	Apr. 06, 2015	Feb. 27, 2015	Feb. 18, 2014	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 03, 2016
Business Acquisition [Line Items]
Acquisition related costs										$ 1,700	$ 1,800	$ 4,200
Goodwill									$ 1,051,294	$ 1,228,503	$ 1,150,244	$ 1,051,294
Concord Systems, Inc.
Business Acquisition [Line Items]
Cash transferred			$ 3,000
Identifiable intangible assets			2,800
Goodwill			$ 1,100
Weighted average useful life			7 years
Soha Systems, Inc.
Business Acquisition [Line Items]
Purchase consideration		$ 55,000
Maximum potential payout of the contingent consideration													$ 5,000
Goodwill, expected tax deductible amount													$ 12,000
Identifiable intangible assets		10,700
Goodwill		$ 43,500
Weighted average useful life		4 years 8 months 12 days
Cyberfend, Inc.
Business Acquisition [Line Items]
Purchase consideration	$ 37,500
Maximum potential payout of the contingent consideration	10,500
Goodwill, expected tax deductible amount	11,000
Identifiable intangible assets	6,500
Goodwill	$ 38,800
Weighted average useful life	3 years 7 months
Xerocole, Inc.
Business Acquisition [Line Items]
Purchase consideration						$ 16,600
Goodwill, expected tax deductible amount						2,700
Identifiable intangible assets						4,900
Goodwill						$ 12,900
Weighted average useful life						8 years 9 months 18 days
Octoshape
Business Acquisition [Line Items]
Purchase consideration					$ 107,047
Measurement period adjustment, increase (decrease) in goodwill								$ (500)
Goodwill, expected tax deductible amount					69,400
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)
Current deferred revenue					(770)
Deferred tax liabilities					(9,858)
Total liabilities assumed					(12,611)
Net assets acquired					107,047
Gross carrying amount of intangible assets					$ 41,950
Weighted average useful life					10 years 7 months 6 days
Octoshape | Completed technologies
Business Acquisition [Line Items]
Gross carrying amount of intangible assets					$ 25,310
Weighted average useful life					9 years 9 months
Octoshape | Customer-related intangible assets
Business Acquisition [Line Items]
Gross carrying amount of intangible assets					$ 16,560
Weighted average useful life					11 years 9 months
Octoshape | Non-compete agreements
Business Acquisition [Line Items]
Gross carrying amount of intangible assets					$ 80
Weighted average useful life					2 years
Bloxx
Business Acquisition [Line Items]
Purchase consideration				$ 18,700
Goodwill, expected tax deductible amount				17,700
Identifiable intangible assets				3,900
Goodwill				$ 17,700
Weighted average useful life				7 years 2 months 12 days
Prolexic
Business Acquisition [Line Items]
Purchase consideration							$ 392,104
Measurement period adjustment, increase (decrease) in goodwill									$ 2,200
Goodwill, expected tax deductible amount							62,400
Cash							33,072
Accounts receivable							11,208
Property and equipment							12,225
Identifiable intangible assets							87,040
Goodwill							293,926
Deferred tax assets							16,340
Other current and long-term assets							5,664
Total assets acquired							459,475
Other current liabilities							(5,940)
Current deferred revenue							(5,812)
Deferred tax liabilities							(36,203)
Other liabilities assumed							(19,416)
Total liabilities assumed							(67,371)
Net assets acquired							392,104
Gross carrying amount of intangible assets							$ 87,040
Weighted average useful life							9 years 2 months 12 days
Prolexic | Completed technologies
Business Acquisition [Line Items]
Gross carrying amount of intangible assets							$ 26,800
Weighted average useful life							6 years 11 months
Prolexic | Customer-related intangible assets
Business Acquisition [Line Items]
Gross carrying amount of intangible assets							$ 58,500
Weighted average useful life							10 years 5 months
Prolexic | Non-compete agreements
Business Acquisition [Line Items]
Gross carrying amount of intangible assets							$ 940
Weighted average useful life							3 years
Prolexic | Trademarks and trade names
Business Acquisition [Line Items]
Gross carrying amount of intangible assets							$ 800
Weighted average useful life							4 years 11 months