Acquisitions - Summary of Fair Value of Aggregate Consideration Paid or Payable (Parenthetical) (Detail)			3 Months Ended	12 Months Ended
	Jan. 03, 2019 USD ($)	Jul. 06, 2018 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 06, 2018 CHF (SFr)
Business Acquisition [Line Items]
Net cash paid					$ 237,684,000	$ 9,250,000
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Cash acquired	$ 7,900,000
Net cash paid	237,700,000
Working capital adjustment	(1,259,000)
Purchase price paid	$ 246,801,000
FeedbackNow [Member]
Business Acquisition [Line Items]
Cash acquired		$ 500,000
Net cash paid		8,000,000
Working capital adjustment		798,000			800,000
Purchase price paid		8,425,000			500,000
Holdback amount expected to be paid		1,485,000
Additional consideration paid to sellers based on financial performance			$ 1,800,000
Business acquisition undiscounted amounts could be payable, minimum		0
Business acquisition undiscounted amounts could be payable, maximum		2,500,000			4,300,000		SFr 4,200,000
Contingent purchase price		$ 3,400,000
FeedbackNow [Member] | Scenario, Forecast [Member]
Business Acquisition [Line Items]
Holdback amount expected to be paid				$ 1,000,000
FeedbackNow [Member] | Maximum [Member]
Business Acquisition [Line Items]
Additional consideration payable to sellers based on financial performance					$ 1,800,000
FeedbackNow [Member] | Maximum [Member] | Scenario, Forecast [Member]
Business Acquisition [Line Items]
Additional consideration payable to sellers based on financial performance				$ 2,500,000