Acquisitions - Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2017	May 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 02, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,478,257		$ 1,385,995	$ 1,385,115
Thinklogical Holdings LLC [Member]
Business Acquisition [Line Items]
Cash		$ 5,376
Receivables		4,355
Inventory		16,424
Prepaid and other current assets		320
Property, plant, and equipment		4,289
Goodwill		71,394
Intangible assets		73,400
Total assets		175,558
Accounts payable		1,231
Accrued liabilities		1,353
Deferred revenue		1,702
Total liabilities		4,286
Net assets		$ 171,272
Tripwire [Member]
Business Acquisition [Line Items]
Cash					$ 2,364
Receivables					37,792
Inventory					603
Other current assets					2,453
Property, plant, and equipment					10,021
Goodwill					462,215
Intangible assets					306,000
Other non-current assets					659
Total assets					822,107
Accounts payable					3,142
Accrued liabilities					12,142
Deferred revenue					8,000
Deferred income taxes					95,074
Other non-current liabilities					540
Total liabilities					118,898
Net assets					$ 703,209