Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) division	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) division	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 713,363	$ 669,628	$ 662,759	$ 668,724	$ 658,470	$ 624,440	$ 605,832	$ 600,293	$ 2,714,474	$ 2,489,035	$ 2,347,988
Number of divisions | division	2								2
Web Division
Disaggregation of Revenue [Line Items]
Revenue									$ 1,446,052	1,305,401	1,138,492
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Revenue									1,268,422	1,183,634	1,209,496
United States
Disaggregation of Revenue [Line Items]
Revenue									1,683,272	1,637,198	1,629,298
International
Disaggregation of Revenue [Line Items]
Revenue									$ 1,031,202	$ 851,837	$ 718,690
Geographic Concentration Risk | Sales | United States
Disaggregation of Revenue [Line Items]
Concentration risk percent (more than)									10.00%	10.00%	10.00%