Acquisitions - Assets Acquired and Liabilities Assumed and Divested (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 12, 2018	Oct. 03, 2018	Jun. 28, 2018	Jun. 27, 2018	Mar. 21, 2018
Assets Acquired
Goodwill	$ 229,319	$ 225,322
Adestra
Assets Acquired
Cash			$ 145
Accounts receivable			2,814
Other current assets			1,395
Property and equipment			796
Goodwill			28,932
Other assets			0
Total assets acquired			68,335
Liabilities Assumed
Accounts payable			(543)
Accrued expense and other			(1,758)
Deferred tax liabilities			(5,104)
Deferred revenue			(1,256)
Total liabilities assumed			(8,661)
Total consideration			59,674
Adestra | Customer relationships
Assets Acquired
Intangible assets			27,542
Adestra | Trade name
Assets Acquired
Intangible assets			710
Adestra | Technology
Assets Acquired
Intangible assets			6,001
Adestra | Noncompetes
Assets Acquired
Intangible assets			$ 0
Rant & Rave
Assets Acquired
Cash				$ 696
Accounts receivable				3,468
Other current assets				3,836
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)
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 | Noncompetes
Assets Acquired
Intangible assets				$ 0
RO Innovation
Assets Acquired
Cash						$ 197
Accounts receivable						1,563
Other current assets						1,299
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)
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 | Noncompetes
Assets Acquired
Intangible assets					$ 1,148
Interfax
Assets Acquired
Cash							$ 1,396
Accounts receivable							1,587
Other current assets							1,341
Property and equipment							286
Goodwill	$ 83,000						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
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 | Noncompetes
Assets Acquired
Intangible assets							$ 0