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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Schedule of Stock-Based Compensation Expense by Operating Expense Category | Total stock-based compensation expense, by operating expense category, recorded in the consolidated statements of operations was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2015 |  |  | 2016 |  |  | 2017 |
Platform operations |  | $ | 71 |  |  | $ | 756 |  |  | $ | 2,674 |
Sales and marketing |  |  | 127 |  |  |  | 1,707 |  |  |  | 6,261 |
Technology and development |  |  | 85 |  |  |  | 1,513 |  |  |  | 6,661 |
General and administrative |  |  | 91 |  |  |  | 1,080 |  |  |  | 5,721 |
Total |  | $ | 374 |  |  | $ | 5,056 |  |  | $ | 21,317 |
Summary of Stock Option Activity | The following summarizes stock option activity:
|  | Shares Under Option (in thousands) |  |  | Weighted- Average Exercise Price |  |  | Weighted-Average Contractual Life (years) |  |  | Aggregate Intrinsic Value (in thousands) |
Outstanding as of December 31, 2016 (1) |  |  | 5,429 |  |  | $ | 4.94 |  |  |  |  |  |  |  |  |
Granted |  |  | 1,473 |  |  |  | 44.20 |  |  |  |  |  |  |  |  |
Exercised |  |  | (1,932 | ) |  |  | 1.33 |  |  |  |  |  |  |  |  |
Cancelled |  |  | (225 | ) |  |  | 18.49 |  |  |  |  |  |  |  |  |
Outstanding as of December 31, 2017 (2) |  |  | 4,745 |  |  | $ | 17.96 |  |  |  | 7.4 |  |  | $ | 136,085 |
Exercisable as of December 31, 2017 |  |  | 2,242 |  |  | $ | 4.81 |  |  |  | 5.9 |  |  | $ | 91,829 |
(1) | Includes options to purchase 389 and 5,040 shares of Class A and Class B common stock, respectively.
(2) | Includes options to purchase 1,780 and 2,965 shares of Class A and Class B common stock, respectively.
Schedule of Weighted-Average Assumptions Used to Value Options Granted to Employees | The fair value of options on the date of grant is estimated based on the Black-Scholes option pricing model. The weighted average assumptions used to value options granted to employees for the periods presented were as follows:
|  | Year Ended December 31, |
|  | 2015 |  |  | 2016 |  |  | 2017 |
Expected term (years) |  |  | 6.0 |  |  |  | 6.0 |  |  |  | 6.0 |
Expected volatility |  |  | 64.5 | % |  |  | 58.1 | % |  |  | 52.6 | %
Risk-free interest rate |  |  | 1.62 | % |  |  | 1.62 | % |  |  | 2.03 | %
Estimated dividend yield |  |  | — | % |  |  | — | % |  |  | — | %
Schedule of Nonvested Restricted Stock Activity | The following summarizes restricted stock activity:
|  | Shares (in thousands) |  |  | Weighted- Average Grant Date Fair Value Per Share |
Unvested as of December 31, 2016 |  |  | 193 |  |  | $ | 29.65 |
Granted |  |  | 307 |  |  |  | 43.76 |
Vested |  |  | (54 | ) |  |  | 30.06 |
Forfeited |  |  | (28 | ) |  |  | 29.97 |
Unvested as of December 31, 2017 |  |  | 418 |  |  | $ | 39.95 |
Schedule of Weighted-Average Assumptions Used to Estimate the Fair Value of ESPP Shares | The fair value of ESPP shares was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions:
|  | Year ended December 31, |
|  | 2016 |  |  | 2017 |
Expected term (years) |  |  | 0.8 |  |  |  | 0.9 |
Expected volatility |  |  | 48.9 | % |  |  | 45.9 | %
Risk-free interest rate |  |  | 0.69 | % |  |  | 1.22 | %
Estimated dividend yield |  |  | — | % |  |  | — | %

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- Definition Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 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 S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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