Fair Value Measurements Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 747,100	$ 790,500
Business Combination, Contingent Consideration, Liability	13,661	30,600
Other Payments to Acquire Businesses	20,000
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	1,100
Ookla [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	40,000
Business Combination, Contingent Consideration, Liability	12,300	25,000
Other Payments to Acquire Businesses	17,000
Payments to Employees	3,000
Salesify [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	17,000
Business Combination, Contingent Consideration, Liability	1,300	$ 5,600
Other Payments to Acquire Businesses	$ 1,000