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Stock-Based Compensation - Stock Options (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Stock Options, additional disclosures

Stock-based compensation expense

$ 21,317

$ 5,056

$ 374

Weighted average assumptions used to value options granted to employees

Estimated dividend yield

0.00%

Stock Options

Shares Under Option

Outstanding at the beginning of the period (in shares)

^[1]

5,429

Granted (in shares)

1,473

Exercised (in shares)

(1,932)

Cancelled (in shares)

(225)

Outstanding at the end of the period (in shares)

4,745

^[2]

5,429

^[1]

Exercisable as of December 31, 2017

2,242

Weighted-Average Exercise Price

Outstanding at the beginning of the period (in dollars per share)

^[1]

$ 4.94

Granted (in dollars per share)

44.20

Exercised (in dollars per share)

1.33

Cancelled (in dollars per share)

18.49

Outstanding at the end of the period (in dollars per share)

17.96

^[2]

$ 4.94

^[1]

Exercisable as of December 31, 2017

$ 4.81

Stock Options, additional disclosures

Weighted-Average Contractual Life, outstanding

^[2]

7 years 4 months 24 days

Weighted-Average Contractual Life, exercisable

5 years 10 months 24 days

Aggregate Intrinsic Value, Outstanding

^[2]

$ 136,085

Aggregate Intrinsic Value, Exercisable

91,829

Stock-based compensation expense

7,900

$ 1,700

$ 400

Unrecognized employee stock-based compensation

$ 36,700

Unrecognized employee stock-based compensation, recognition period

2 years 9 months 18 days

Weighted average assumptions used to value options granted to employees

Expected term (years)

6 years

6 years

6 years

Volatility (as a percent)

52.60%

58.10%

64.50%

Risk-free interest rate (as a percent)

2.03%

1.62%

1.62%

Estimated dividend yield

0.00%

0.00%

0.00%

Weighted average grant date fair value per share

$ 22.48

$ 11.61

$ 1.12

Total intrinsic value of options exercised

$ 84,800

$ 13,700

$ 1,900

[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.

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