Acquisitions - Assets Acquired and Liabilities Assumed and Divested (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Aug. 21, 2019	May 24, 2019	Apr. 18, 2019	Dec. 31, 2018	Dec. 12, 2018	Oct. 03, 2018	Jun. 27, 2018	Mar. 21, 2018
Assets Acquired
Operating lease right-of-use asset	$ 5,692				$ 0
Goodwill	282,833				$ 225,322
Liabilities Assumed
Operating lease liabilities	(6,030)
Cimpl
Assets Acquired
Cash		$ 137
Accounts receivable		1,115
Other current assets		1,775
Operating lease right-of-use asset		0
Property and equipment		118
Goodwill		12,778
Other assets		6
Total assets acquired		31,680
Liabilities Assumed
Accounts payable		(302)
Accrued expense and other		(1,168)
Deferred tax liabilities		(4,191)
Deferred revenue		(348)
Operating lease liabilities		0
Total liabilities assumed		(6,009)
Total consideration		25,671
Cimpl | Customer relationships
Assets Acquired
Intangible assets		12,181
Cimpl | Trade name
Assets Acquired
Intangible assets		375
Cimpl | Technology
Assets Acquired
Intangible assets		3,195
Cimpl | Noncompete agreements
Assets Acquired
Intangible assets		$ 0
Adestra
Assets Acquired
Cash						$ 145
Accounts receivable						2,774
Other current assets						1,395
Operating lease right-of-use asset						0
Property and equipment						796
Goodwill						29,218
Other assets						0
Total assets acquired						68,580
Liabilities Assumed
Accounts payable						(543)
Accrued expense and other						(1,723)
Deferred tax liabilities						(5,104)
Deferred revenue						(1,536)
Operating lease liabilities						0
Total liabilities assumed						(8,906)
Total consideration						59,674
Adestra | Customer relationships
Assets Acquired
Intangible assets						27,542
Adestra | Trade name
Assets Acquired
Intangible assets						709
Adestra | Technology
Assets Acquired
Intangible assets						6,001
Adestra | Noncompete agreements
Assets Acquired
Intangible assets						$ 0
Rant & Rave
Assets Acquired
Cash							$ 696
Accounts receivable							3,468
Other current assets							3,836
Operating lease right-of-use asset							0
Property and equipment							131
Goodwill							32,589
Other assets							0
Total assets acquired							78,365
Liabilities Assumed
Accounts payable							(1,577)
Accrued expense and other							(6,114)
Deferred tax liabilities							(3,896)
Deferred revenue							(2,019)
Operating lease liabilities							0
Total liabilities assumed							(13,606)
Total consideration							64,759
Rant & Rave | Customer relationships
Assets Acquired
Intangible assets							29,981
Rant & Rave | Trade name
Assets Acquired
Intangible assets							1,099
Rant & Rave | Technology
Assets Acquired
Intangible assets							6,565
Rant & Rave | Noncompete agreements
Assets Acquired
Intangible assets							$ 0
Kapost
Assets Acquired
Cash			$ 0
Accounts receivable			3,906
Other current assets			1,101
Operating lease right-of-use asset			2,136
Property and equipment			687
Goodwill			21,542
Other assets			0
Total assets acquired			59,650
Liabilities Assumed
Accounts payable			(64)
Accrued expense and other			(1,689)
Deferred tax liabilities			(2,483)
Deferred revenue			(3,879)
Operating lease liabilities			(2,136)
Total liabilities assumed			(10,251)
Total consideration			49,399
Kapost | Customer relationships
Assets Acquired
Intangible assets			23,735
Kapost | Trade name
Assets Acquired
Intangible assets			787
Kapost | Technology
Assets Acquired
Intangible assets			5,756
Kapost | Noncompete agreements
Assets Acquired
Intangible assets			$ 0
Postup Holdings
Assets Acquired
Cash				$ 19
Accounts receivable				1,043
Other current assets				1,373
Operating lease right-of-use asset				0
Property and equipment				743
Goodwill				22,105
Other assets				0
Total assets acquired				39,361
Liabilities Assumed
Accounts payable				(448)
Accrued expense and other				(530)
Deferred tax liabilities				(3,368)
Deferred revenue				(15)
Operating lease liabilities				0
Total liabilities assumed				(4,361)
Total consideration				35,000
Postup Holdings | Customer relationships
Assets Acquired
Intangible assets				10,667
Postup Holdings | Trade name
Assets Acquired
Intangible assets				468
Postup Holdings | Technology
Assets Acquired
Intangible assets				2,943
Postup Holdings | Noncompete agreements
Assets Acquired
Intangible assets				$ 0
RO Innovation
Assets Acquired
Cash								$ 197
Accounts receivable								1,563
Other current assets								1,299
Operating lease right-of-use asset								0
Property and equipment								15
Goodwill								7,568
Other assets								0
Total assets acquired								20,259
Liabilities Assumed
Accounts payable								(229)
Accrued expense and other								(1,921)
Deferred tax liabilities								(2,129)
Deferred revenue								(1,817)
Operating lease liabilities								0
Total liabilities assumed								(6,096)
Total consideration								14,163
RO Innovation | Customer relationships
Assets Acquired
Intangible assets								6,688
RO Innovation | Trade name
Assets Acquired
Intangible assets								111
RO Innovation | Technology
Assets Acquired
Intangible assets								1,670
RO Innovation | Noncompete agreements
Assets Acquired
Intangible assets								$ 1,148
Interfax
Assets Acquired
Cash									$ 1,396
Accounts receivable									1,587
Other current assets									1,341
Operating lease right-of-use asset									0
Property and equipment									286
Goodwill	$ 139,700								13,862
Other assets									14
Total assets acquired									46,948
Liabilities Assumed
Accounts payable									(737)
Accrued expense and other									(2,847)
Deferred tax liabilities									(3,364)
Deferred revenue									0
Operating lease liabilities									0
Total liabilities assumed									(6,948)
Total consideration									40,000
Interfax | Customer relationships
Assets Acquired
Intangible assets									22,577
Interfax | Trade name
Assets Acquired
Intangible assets									649
Interfax | Technology
Assets Acquired
Intangible assets									5,236
Interfax | Noncompete agreements
Assets Acquired
Intangible assets									$ 0