Revenue from Contracts with Customers - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($) division	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) division	Jun. 30, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 662,759	$ 605,832	$ 1,331,483	$ 1,206,125
Number of divisions \| division	2		2
Web Division
Disaggregation of Revenue [Line Items]
Revenue	$ 351,084	317,408	$ 703,921	620,896
Media and Carrier Division
Disaggregation of Revenue [Line Items]
Revenue	311,675	288,424	627,562	585,229
U.S.
Disaggregation of Revenue [Line Items]
Revenue	413,129	400,236	836,468	799,106
International
Disaggregation of Revenue [Line Items]
Revenue	$ 249,630	$ 205,596	$ 495,015	$ 407,019

Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Name:	us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration