Summary of Significant Accounting Policies - Additional Information (Detail)			12 Months Ended
	Jan. 01, 2019 USD ($)	Jan. 02, 2018 USD ($)	Dec. 31, 2019 USD ($) ConsumerandBusinessLeader Customer shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 03, 2019 USD ($)
Summary Of Significant Accounting Policy [Line Items]
Operating lease right-of-use assets			$ 69,100,000
Operating lease liabilities			$ 79,270,000
Lease package practical expedients			true
Reclassification adjustment to retained earnings for tax effects				$ 3,803,000	$ (419,000)
Deferred revenue			$ 179,194,000	135,332,000
Prepaid expenses and other current assets			19,201,000	12,802,000
Deferred commissions			20,326,000	15,677,000
Other assets			6,829,000	5,310,000
Retained earnings			$ 118,147,000	127,717,000
Original maturities			90 days
Realized losses on sales of available-for-sale securities				200,000
Number of customers accounted for revenues or accounts receivable greater than 1% or 1% of total | Customer			0
Annual goodwill impairment test, period			November 30th
Goodwill impairment charges	$ 0		$ 0	0	0
Non-current deferred rent liabilities				6,602,000
Foreign exchange losses			(933,000)	(603,000)	(632,000)
Contract assets			$ 0
Contract with customer, contract duration			1 year
Deferred revenue recognized			$ 147,200,000	134,700,000
Revenue expected to be recognized			372,000,000
Amortization expense related to deferred commissions			$ 36,000,000	32,200,000
Option to extend description			Some of the Company’s leases include options to extend or terminate the lease. When determining the lease term, these options are included in the measurement and recognition of the Company’s ROU assets and lease liabilities when it is reasonably certain that the Company will exercise the option.
Advertising expense			$ 1,300,000	600,000	$ 700,000
Stock options granted | shares			0
Unamortized fair value stock based compensation				$ 21,200,000
Weighted average remaining recognition period			2 years 3 months 18 days
Research Service Revenue [Member] | Customer Concentration Risk [Member]
Summary Of Significant Accounting Policy [Line Items]
Customer accounted for percentage			4.00%
Accounts Receivable [Member] | Credit Concentration Risk [Member]
Summary Of Significant Accounting Policy [Line Items]
Customer accounted for percentage			4.00%
Topic 842 [Member]
Summary Of Significant Accounting Policy [Line Items]
Operating lease right-of-use assets	53,300,000
Operating lease liabilities	60,800,000
Topic 842 [Member] | Effect of Adoption [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred rent	$ 7,500,000
Topic 842 [Member] | SiriusDecisions, Inc [Member]
Summary Of Significant Accounting Policy [Line Items]
Operating lease right-of-use assets						$ 10,400,000
Operating lease liabilities						$ 10,400,000
ASU No. 2018-02 [Member]
Summary Of Significant Accounting Policy [Line Items]
Reclassification adjustment to retained earnings for tax effects		$ 26,000
Topic 606 [Member] | Initial Application Period Cumulative Effect Transition [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred revenue			$ 7,800,000
Prepaid expenses and other current assets			(5,500,000)
Deferred commissions			900,000
Other assets			600,000
Retained earnings			$ 3,800,000
Minimum [Member]
Summary Of Significant Accounting Policy [Line Items]
Number of consumers and business leaders included in annual surveys | ConsumerandBusinessLeader			690,000
Advisory services period			1 hour
Consulting services period			14 days
Maximum [Member]
Summary Of Significant Accounting Policy [Line Items]
Advisory services period			2 days
Consulting services period			3 months
Contract with customer, expected payment term			one year