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Summary of Significant Accounting Policies - Additional Information (Detail) |  |  | 3 Months Ended | 12 Months Ended
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Jan. 03, 2019 USD ($) | Jan. 02, 2018 USD ($) | Dec. 31, 2018 USD ($) $ / shares | Sep. 30, 2018 USD ($) $ / shares | Jun. 30, 2018 USD ($) $ / shares | Mar. 31, 2018 USD ($) $ / shares | Dec. 31, 2017 USD ($) $ / shares | Sep. 30, 2017 USD ($) $ / shares | Jun. 30, 2017 USD ($) $ / shares | Mar. 31, 2017 USD ($) $ / shares | Dec. 31, 2018 USD ($) ConsumerandBusinessLeader Customer $ / shares shares | Dec. 31, 2017 USD ($) $ / shares | Dec. 31, 2016 USD ($) $ / shares
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment to retained earnings for tax effects |  |  |  |  |  |  |  |  |  |  | $ 3,803,000 | $ (419,000) |
Deferred revenue |  |  | $ 135,332,000 |  |  |  | $ 145,207,000 |  |  |  | 135,332,000 | 145,207,000 |
Prepaid expenses and other current assets |  |  | 12,802,000 |  |  |  | 18,942,000 |  |  |  | 12,802,000 | 18,942,000 |
Deferred commissions |  |  | 15,677,000 |  |  |  | 13,731,000 |  |  |  | 15,677,000 | 13,731,000 |
Other assets |  |  | 5,310,000 |  |  |  | 6,231,000 |  |  |  | 5,310,000 | 6,231,000 |
Retained earnings |  |  | 127,717,000 |  |  |  | 123,010,000 |  |  |  | 127,717,000 | 123,010,000 |
Total assets |  |  | 353,524,000 |  |  |  | 345,200,000 |  |  |  | 353,524,000 | 345,200,000 |
Total revenues |  |  | 98,583,000 | $ 84,890,000 | $ 96,353,000 | $ 77,749,000 | 90,377,000 | $ 80,369,000 | $ 89,733,000 | $ 77,194,000 | 357,575,000 | 337,673,000 | $ 326,095,000
Net income |  |  | $ 5,375,000 | $ 3,950,000 | $ 7,788,000 | $ (1,733,000) | $ 2,093,000 | $ 3,953,000 | $ 6,064,000 | $ 3,030,000 | $ 15,380,000 | $ 15,140,000 | $ 17,651,000
Diluted income per common share | $ / shares |  |  | $ 0.29 | $ 0.21 | $ 0.43 | $ (0.10) | $ 0.11 | $ 0.22 | $ 0.34 | $ 0.16 | $ 0.84 | $ 0.83 | $ 0.97
Original maturities |  |  |  |  |  |  |  |  |  |  | 90 days |  |
Number of customers accounted for revenues or accounts receivable greater than 3% or 6% of total | Customer |  |  |  |  |  |  |  |  |  |  | 0 |  |
Annual goodwill impairment test, period |  |  |  |  |  |  |  |  |  |  | November 30th |  |
Goodwill impairment charges |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0
Non-current deferred rent liabilities |  |  | $ 6,602,000 |  |  |  | $ 7,523,000 |  |  |  | 6,602,000 | 7,523,000 |
Foreign exchange gains (losses) |  |  |  |  |  |  |  |  |  |  | (603,000) | $ (632,000) | $ 81,000
Contract assets |  |  | 0 |  |  |  |  |  |  |  | $ 0 |  |
Contract with customer, contract duration |  |  |  |  |  |  |  |  |  |  | 1 year |  |
Deferred revenue recognized |  |  |  |  |  |  |  |  |  |  | $ 134,700,000 |  |
Revenue expected to be recognized |  |  | 262,600,000 |  |  |  |  |  |  |  | 262,600,000 |  |
Amortization expense related to deferred commissions |  |  |  |  |  |  |  |  |  |  | $ 32,200,000 |  |
Options granted | shares |  |  |  |  |  |  |  |  |  |  | 0 |  |
Unamortized fair value stock based compensation |  |  | 17,100,000 |  |  |  |  |  |  |  | $ 17,100,000 |  |
Weighted average remaining recognition period |  |  |  |  |  |  |  |  |  |  | 2 years 6 months |  |
Research Service Revenue [Member] | Customer Concentration Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Customer accounted for percentage |  |  |  |  |  |  |  |  |  |  | 3.00% |  |
Accounts Receivable [Member] | Credit Concentration Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Customer accounted for percentage |  |  |  |  |  |  |  |  |  |  | 6.00% |  |
ASU No. 2018-02 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment to retained earnings for tax effects |  | $ 26,000 |  |  |  |  |  |  |  |  |  |  |
ASC 606 [Member] | Initial Application Period Cumulative Effect Transition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred revenue |  |  | 7,800,000 |  |  |  |  |  |  |  | $ 7,800,000 |  |
Prepaid expenses and other current assets |  |  | (5,500,000) |  |  |  |  |  |  |  | (5,500,000) |  |
Deferred commissions |  |  | 900,000 |  |  |  |  |  |  |  | 900,000 |  |
Other assets |  |  | 600,000 |  |  |  |  |  |  |  | 600,000 |  |
Retained earnings |  |  | 3,800,000 |  |  |  |  |  |  |  | 3,800,000 |  |
ASC 606 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred revenue |  |  | (14,000,000) |  |  |  |  |  |  |  | (14,000,000) |  |
Total assets |  |  | (8,800,000) |  |  |  |  |  |  |  | (8,800,000) |  |
Total revenues |  |  |  |  |  |  |  |  |  |  | (1,600,000) |  |
Net income |  |  |  |  |  |  |  |  |  |  | $ (1,400,000) |  |
Diluted income per common share | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 0.08 |  |
SiriusDecisions, Inc [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of issued and outstanding shares acquired | 100.00% |  |  |  |  |  |  |  |  |  |  |  |
Cash paid | $ 247,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of consumers and business leaders included in annual surveys | ConsumerandBusinessLeader |  |  |  |  |  |  |  |  |  |  | 675,000 |  |
Advisory services period |  |  |  |  |  |  |  |  |  |  | 1 hour |  |
Consulting services period |  |  |  |  |  |  |  |  |  |  | 14 days |  |
Minimum [Member] | ASU No. 2016-02 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Right-of-use asset |  |  | 50,000,000 |  |  |  |  |  |  |  | $ 50,000,000 |  |
Liability |  |  | 58,000,000 |  |  |  |  |  |  |  | $ 58,000,000 |  |
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 |  |
Maximum [Member] | ASU No. 2016-02 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Right-of-use asset |  |  | 54,000,000 |  |  |  |  |  |  |  | $ 54,000,000 |  |
Liability |  |  | $ 62,000,000 |  |  |  |  |  |  |  | $ 62,000,000 |  |

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