Significant Accounting Policies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) customer	Dec. 31, 2016 USD ($) customer distributor	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts	$ 3,400	$ 3,900
Estimated useful lives of intangible assets	6 years 2 months 12 days	6 years 2 months 23 days
Amount capitalized related to internal use software	$ 41,500	$ 29,200
Amount expensed related to internal use software	27,300	37,800	$ 36,100
Capitalized costs for internally developed software to be sold as a service	15,200	18,200
Amount expensed related to internally developed computer software to be sold as a service	18,500	16,800	9,200
Distribution of the GoTo Business	13,400
Allowance for estimated product returns	1,200	2,000
Advertising costs	85,600	72,800	70,700
Pension Plan | Foreign Plan
Significant Accounting Policies [Line Items]
Termination indemnities	13,200	13,200
Termination indemnities, compensation expense	$ 2,600	$ 2,500	$ 3,800
Minimum
Significant Accounting Policies [Line Items]
Estimated useful lives of intangible assets	3 years
Licensing agreement term	1 year
Minimum | License update
Significant Accounting Policies [Line Items]
Revenue recognition, period for recognition	12 months
Maximum
Significant Accounting Policies [Line Items]
Estimated useful lives of intangible assets	7 years
Licensing agreement term	5 years
Maximum | License update
Significant Accounting Policies [Line Items]
Revenue recognition, period for recognition	24 months
Customer concentration risk | Accounts receivable
Significant Accounting Policies [Line Items]
Number of distributors meeting concentration risk threshold | distributor		2
Number of customers meeting concentration risk threshold | customer	1	2
Internal use software | Minimum
Significant Accounting Policies [Line Items]
Estimated useful lives of intangible assets	3 years
Internal use software | Maximum
Significant Accounting Policies [Line Items]
Estimated useful lives of intangible assets	7 years
Internally developed software to be sold as a service
Significant Accounting Policies [Line Items]
Estimated useful lives of intangible assets	2 years
Arrow Group | Customer concentration risk | Accounts receivable
Significant Accounting Policies [Line Items]
Concentration risk percentage	14.00%	14.00%
Ingram Micro | Customer concentration risk | Accounts receivable
Significant Accounting Policies [Line Items]
Concentration risk percentage		10.00%
Foreign currency
Significant Accounting Policies [Line Items]
Distribution of the GoTo Business	$ 13,400