Acquisitions - Summary of Allocation of Purchase Price to Fair Value of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Jan. 03, 2019	Dec. 31, 2018	Jul. 06, 2018
Assets:
Goodwill	$ 247,211	$ 243,895		$ 85,165
SiriusDecisions, Inc [Member]
Assets:
Cash and cash equivalents			$ 7,858
Accounts receivable			19,237
Prepaids and other current assets			3,660
Property and equipment			4,169
Goodwill			158,569
Intangible assets			115,000
Other assets			418
Total assets			308,911
Liabilities:
Accounts payable and other current liabilities			8,924
Deferred revenue			26,143
Deferred tax liability			26,226
Long-term deferred revenue			1,037
Other long-term liabilities			1,039
Total liabilities			63,369
Net assets acquired			$ 245,542
FeedbackNow [Member]
Assets:
Cash and cash equivalents					$ 463
Accounts receivable					738
Prepaids and other current assets					487
Goodwill					9,513
Intangible assets					4,780
Other assets					75
Total assets					16,056
Liabilities:
Accounts payable and other current liabilities					837
Contract liabilities					298
Deferred tax liability					825
Total liabilities					1,960
Net assets acquired					$ 14,096