Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) division	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) division	Sep. 30, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 709,912	$ 669,628	$ 2,121,494	$ 2,001,111
Number of divisions | division	2		2
Web Division
Disaggregation of Revenue [Line Items]
Revenue	$ 390,223	357,255	$ 1,146,872	1,062,376
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Revenue	319,689	312,373	974,622	938,735
U.S.
Disaggregation of Revenue [Line Items]
Revenue	413,116	412,573	1,248,175	1,249,041
International
Disaggregation of Revenue [Line Items]
Revenue	$ 296,796	$ 257,055	$ 873,319	$ 752,070