Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) division	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) division	Jun. 30, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 705,074	$ 662,759	$ 1,411,582	$ 1,331,483
Number of divisions | division	2		2
Web Division
Disaggregation of Revenue [Line Items]
Revenue	$ 380,374	351,871	$ 756,649	705,121
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Revenue	324,700	310,888	654,933	626,362
U.S.
Disaggregation of Revenue [Line Items]
Revenue	416,859	413,129	835,059	836,468
International
Disaggregation of Revenue [Line Items]
Revenue	$ 288,215	$ 249,630	$ 576,523	$ 495,015