Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) division	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) division	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 846,287	$ 792,845	$ 794,715	$ 764,302	$ 772,123	$ 709,912	$ 705,074	$ 706,508	$ 3,198,149	$ 2,893,617	$ 2,714,474
Number of divisions | division	2								2
Web Division
Disaggregation of Revenue [Line Items]
Revenue									$ 1,666,305	1,556,252	1,439,772
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Revenue									1,531,844	1,337,365	1,274,702
U.S.
Disaggregation of Revenue [Line Items]
Revenue									1,777,435	1,694,211	1,683,272
International
Disaggregation of Revenue [Line Items]
Revenue									$ 1,420,714	$ 1,199,406	$ 1,031,202