Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) division	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) division	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 772,123	$ 709,912	$ 705,074	$ 706,508	$ 713,363	$ 669,628	$ 662,759	$ 688,724	$ 2,893,617	$ 2,714,474	$ 2,489,035
Number of divisions | division	2								2
Web Division
Disaggregation of Revenue [Line Items]
Revenue									$ 1,566,401	1,448,644	1,307,641
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Revenue									1,327,216	1,265,830	1,181,394
U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,694,211	1,683,272	1,637,198
International
Disaggregation of Revenue [Line Items]
Revenue									$ 1,199,406	$ 1,031,202	$ 851,837
Geographic Concentration Risk | Sales | U.S.
Disaggregation of Revenue [Line Items]
Concentration risk percent (more than)									10.00%	10.00%	10.00%