Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) division	Mar. 31, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Total revenue	$ 668,724	$ 600,293
Number of divisions | division	2
Web Division
Disaggregation of Revenue [Line Items]
Total revenue	$ 352,837	303,488
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Total revenue	315,887	296,805
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	423,339	398,870
International
Disaggregation of Revenue [Line Items]
Total revenue	$ 245,385	$ 201,423