Stock-Based Compensation - Additional Information (Detail) $ in Thousands	6 Months Ended
Jun. 30, 2019 USD ($) shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting terms	vesting terms ranging from three to five years and include 1) options that vest in one-fifth increments beginning on the grant date and continuing on each of the first four anniversaries of the grant date and 2) options that cliff-vest on the fifth anniversary of the grant date.
Number of shares available for grant	2,714,981
Common Class A [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted	81,000
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	3 years
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	5 years
1996 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for issuance to directors and employees	17,500,000
1996 Equity Incentive Plan [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of awards of target number of share	0.00%
1996 Equity Incentive Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of awards of target number of share	100.00%
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration date of options granted under equity incentive plan	10 years
Restricted Stock [Member] \| Non-employee Director [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense related to performance based compensation agreements \| $	$ 326
Restricted Stock [Member] \| Percentage of awards vesting on grant date [Member] \| Non-employee Director [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of awards vesting on grant date	50.00%
Restricted Stock [Member] \| Percentage of Awards Vesting On Last Day of Each Directors Term [Member] \| Non-employee Director [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	1 year
Percentage of awards vesting on grant date	50.00%
2009 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares reserved	85,162
2019 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares reserved	300,000
Performance Based Compensation [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense related to performance based compensation agreements \| $	$ 4,955

Shares subject to awards as percentage of target number of shares.

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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.

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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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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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Number of additional shares authorized for issuance under an established share-based compensation plan.

+ References

No definition available.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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

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Description of pertinent provisions of equity-based compensation awards that have actual or potential impact upon the company's financial statements.

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Percentage of vesting of share-based compensation awards.

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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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