Stockholders' Equity - Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Expected volatility, maximum (as a percent)	63.28%	60.56%	60.60%
Risk-free interest rate, maximum (as a percent)	2.55%	2.85%	2.18%
Aggregate Intrinsic Value (in thousands)
Outstanding	$ 27,716	$ 57,220	$ 108,520
Exercised	2,423	$ 36,317
Vested and expected to vest	27,716
Exercisable	$ 25,594
Stock option and Stock Appreciation Rights
Number of Options and SAR
Outstanding at the beginning of the period (in shares)	3,916,963	4,280,670
Granted (in shares)	880,235	762,915
Exercised (in shares)	(114,753)	(930,483)
Forfeited (in shares)	(75,886)	(196,139)
Outstanding at the end of the period (in shares)	4,606,559	3,916,963	4,280,670
Vested and expected to vest (in shares)	4,606,559
Exercisable (in shares)	2,598,112
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 19.98	$ 14.50
Granted (in dollars per share)	36.43	39.91
Exercised (in dollars per share)	12.90	10.07
Forfeited (in dollars per share)	34.80	25.01
Outstanding at the end of the period (in dollars per share)	23.05	$ 19.98	$ 14.50
Vested and expected to vest (in dollars per share)	23.05
Exercisable (in dollars per share)	$ 15.68
Stock Option
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Expected volatility, minimum (as a percent)	61.36%	57.95%	53.61%
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected term		6 years 3 months	6 years 3 months
Risk-free interest rate, minimum (as a percent)	1.69%	2.69%	1.90%
Expected forfeiture rate (as a percent)	0.00%	0.00%	0.00%
Weighted-Average Remaining Contractual Term (in years)
Outstanding at the end of the period	6 years 7 months 28 days	7 years 1 month 6 days	7 years 4 months 13 days
Vested and expected to vest	6 years 7 months 28 days
Exercisable	5 years 5 months 23 days
Share-based payments, additional disclosure
Total unrecognized compensation expense	$ 26,300	$ 22,400
Weighted-average period over which total unrecognized compensation expense is expected to be recognized	2 years 6 months 7 days	2 years 7 months 24 days
Weighted-average, grant-date fair value of options granted (in dollars per share)	$ 21.50	$ 23.43	$ 14.35
Total fair value of the common stock vested	$ 10,800	$ 8,300	$ 5,400
Stock Option \| Minimum
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Fair value of common stock (in dollars per share)	$ 22.99	$ 37.20	$ 25.30
Expected term	5 years 6 months 10 days
Stock Option \| Maximum
Assumptions used in estimating the fair value of each option award using the Black-Scholes option-pricing model
Fair value of common stock (in dollars per share)	$ 37.78	$ 58.15	$ 41.00
Expected term	6 years 2 months 4 days

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the maximum 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.

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The estimated measure of the minimum 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.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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