Stock-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	5 years
Granted (in shares)	252,100	653,900	540,548
Weighted average grant date fair value for stock options (USD per share)	$ 14.95	$ 8.77	$ 5.90
Fair value of stock options vested during period	$ 3,500	$ 2,200	$ 2,700
Aggregate intrinsic value of stock options exercised during period	34,999	14,100	3,300
Compensation cost not yet recognized on nonvested awards	4,200
Cash received from exercise of stock options	$ 2,600	$ 1,100	500
Stock options and assumed options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash settlement of exercised stock options included in stock-based compensation			$ 800
Dividend rate (percent)	0.00%	0.00%	0.00%
Compensation cost not yet recognized, period for recognition (in years)	2 years 2 months 12 days
2009 and 2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares of common stock outstanding (shares)	13,281	29,835
2009 and 2015 Plan \| Stock options and assumed options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	5 years
Award expiration period (in years)	10 years
Common Stock \| 2009 and 2015 Plan \| Stock options and assumed options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of unvested shares (shares)	1,492	2,156
Accounts Payable, Accrued Expenses and Other Current Liabilities \| 2009 and 2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Liability from proceeds of early exercise of stock options	$ 100	$ 200

Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements payable within one year (or the operating cycle, if longer).

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Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements.

+ References

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested options awarded to employees as compensation.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen 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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Gross number of share options (or share units) granted during the period.

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No definition available.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of non-vested options outstanding.

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No definition available.

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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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