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Stock-Based Compensation - ESPP (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2016
Weighted-average assumptions used to estimate the fair value of ESPP shares |  |  |  |
Estimated dividend yield | 0.00% |  |  |
Stock-based compensation expense | $ 21,317 | $ 5,056 | $ 374 |
ESPP |  |  |  |
Stock-Based Compensation |  |  |  |
Shares remained available for grant | 500 |  |  |
Maximum annual increase in shares available for grant (in shares) | 800,000 |  |  |
Maximum employee payroll deduction (as a percent) | 100.00% |  |  |
Maximum offering period | 2 years |  |  |
Period between purchases | 6 months |  |  |
Price of ESPP shares as percentage of market price | 85.00% |  |  |
Weighted-average assumptions used to estimate the fair value of ESPP shares |  |  |  |
Expected term (years) | 10 months 24 days | 9 months 18 days |  |
Volatility (as a percent) | 45.90% | 48.90% |  |
Risk-free interest rate (as a percent) | 1.22% | 0.69% |  |
Estimated dividend yield | 0.00% | 0.00% |  |
Holding period for purchases after the first offering period |  | 6 months |  |
Holding period for purchases during the first offering period |  | 12 months |  |
Stock-based compensation expense | $ 10,700 | $ 3,300 |  |
Unrecognized employee stock-based compensation | $ 6,600 |  |  |
Unrecognized employee stock-based compensation, recognition period | 8 months 12 days |  |  |
ESPP | First Offering Period |  |  |  |
Weighted-average assumptions used to estimate the fair value of ESPP shares |  |  |  |
Stock-based compensation expense | $ 10,100 | $ 3,300 |  |
ESPP | Common Stock |  |  |  |
Stock-Based Compensation |  |  |  |
Maximum annual increase in shares available for issuance, percentage of outstanding shares | 1.00% |  |  |
ESPP | Class A common stock |  |  |  |
Stock-Based Compensation |  |  |  |
Stock available for issuance (in shares) |  |  |  | 800,000

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- Definition The maximum annual increase in shares reserved for issuance under the stock-based compensation plan, as a percentage of the entity's outstanding shares on an as-converted basis as of the end of the immediately preceding calendar year. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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