Stock-based Compensation Plans - Additional Information (Detail)	6 Months Ended
Jun. 30, 2018 shares
Class B
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units holder entitle to receive number of shares of common stock upon certain service conditions	1

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.

No definition available.

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