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Summary of Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017 USD ($) Customer shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policy [Line Items] |  |  |
Original maturities | 90 days |  |
Number of customers accounted for revenues or accounts receivable greater than 2% or 3% of total | Customer | 0 |  |
Annual goodwill impairment test, period | November 30th |  |
Goodwill impairment charges | $ 0 | $ 0 | $ 0
Non-current deferred rent liabilities | 7,500,000 | 7,400,000 |
Foreign exchange gains (losses) | (600,000) | 100,000 |
Tax withholding in financing activities | $ 2,527,000 | 2,069,000 | 1,459,000
Options granted | shares | 0 |  |
Unamortized fair value stock based compensation | $ 16,600,000 |  |
Weighted average remaining recognition period | 2 years 7 months 6 days |  |
ASU No. 2016-09 [Member] |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |
Cumulative effect adjustment to increase or (reduce) retained earnings |  | 200,000 |
Decrease to income tax expense | $ 400,000 |  |
Tax withholding in financing activities | $ 2,500,000 |  |
Change in reclassification of tax withholding |  | 2,600,000 | $ 1,500,000
Early Adoption of ASU No. 2016-16 [Member] |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |
Cumulative effect adjustment to increase or (reduce) retained earnings |  | $ (500,000) |
Research Service Revenue [Member] | Customer Concentration Risk [Member] |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |
Customer accounted for percentage | 2.00% |  |
Accounts Receivable [Member] | Credit Concentration Risk [Member] |  |  |
Summary Of Significant Accounting Policy [Line Items] |  |  |
Customer accounted for percentage | 3.00% |  |

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