Acquisitions - Intangible Assets Related to Acquisition (Details) - USD ($) $ in Thousands			12 Months Ended
	May 31, 2017	Jan. 02, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Goodwill			$ 1,478,257	$ 1,385,995	$ 1,385,115
Developed Technology [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Weighted average amortization period			6 years 6 months
Customer Relationships [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Weighted average amortization period			18 years 6 months
Trademarks [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Weighted average amortization period			8 years
Thinklogical Holdings LLC [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization	$ 73,400
Goodwill	71,394
Intangible assets not subject to amortization	71,394
Total intangible assets	$ 144,794
Weighted average amortization period	9 years 7 months 6 days
Thinklogical Holdings LLC [Member] | Developed Technology [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization	$ 62,600
Weighted average amortization period	10 years
Thinklogical Holdings LLC [Member] | Customer Relationships [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization	$ 6,500
Weighted average amortization period	8 years
Thinklogical Holdings LLC [Member] | Trademarks [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization	$ 2,900
Weighted average amortization period	10 years
Thinklogical Holdings LLC [Member] | Sales Backlog [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization	$ 1,400
Weighted average amortization period	3 months 19 days
Tripwire [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization		$ 300,000
Goodwill		462,215
Intangible assets not subject to amortization		468,215
Total intangible assets		$ 768,215
Weighted average amortization period		7 years 11 months
Tripwire [Member] | In-Process Research and Development [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets not subject to amortization		$ 6,000
Tripwire [Member] | Developed Technology [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization		$ 210,000
Weighted average amortization period		5 years 9 months 18 days
Tripwire [Member] | Customer Relationships [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization		$ 56,000
Weighted average amortization period		15 years
Tripwire [Member] | Trademarks [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization		$ 31,000
Weighted average amortization period		10 years	10 years
Tripwire [Member] | Backlog [Member]
Acquired Finite And Indefinite Lived Intangible Assets [Line Items]
Intangible assets subject to amortization		$ 3,000
Weighted average amortization period		1 year