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STOCKHOLDERS' EQUITY - Narrative (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Oct. 30, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2016
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Fair value of stock awards vested |  | $ 41,000 |  | $ 5,892,000 |  |
Stock Options |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | 0 | $ 0 | 0 | $ 0 |
Aggregate intrinsic value of options exercised |  | 0 |  | 296,000 |  |
Restricted Stock, Restricted Stock Units And Performance Stock Units |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | 1,898,000 | 1,858,000 | 5,175,000 | 5,084,000 |
Restricted Stock |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation expense |  | 84,000 |  | $ 84,000 |  |
Restricted Stock | Non Employee Director |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options, vesting period |  |  |  | 3 years |  |
Restricted Stock Unit |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation expense |  | 4,684,000 |  | $ 4,684,000 |  |
Performance Stock Unit |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation expense |  | 5,196,000 |  | $ 5,196,000 |  |
Weighted average | Restricted Stock |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation, weighted average period |  |  |  | 2 years 7 months 6 days |  |
Weighted average | Restricted Stock Unit |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation, weighted average period |  |  |  | 1 year 10 months 24 days |  |
Weighted average | Performance Stock Unit |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation, weighted average period |  |  |  | 1 year 9 months 18 days |  |
2016 Plan |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options granted, term |  |  |  | 10 years |  |
2016 Plan | Minimum |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options, vesting period |  |  |  | 1 year |  |
2016 Plan | Maximum |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options, vesting period |  |  |  | 4 years |  |
Amended and Restated 1997 Employee Stock Purchase Plan |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 67,000 | $ 40,000 | $ 171,000 | $ 134,000 |
Class A Common Stock | Subsequent Event |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Number of shares issued | 2,703,000 |  |  |  |  |
Public offering price (in dollars per share) | $ 56.00 |  |  |  |  |
Net proceeds | $ 144,711,000 |  |  |  |  |
Class A Common Stock | 2016 Plan |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Common stock, authorized shares |  |  |  |  |  | 2,250,000
Number of shares available for future grant |  | 1,102,000 |  | 1,102,000 |  |
Class A Common Stock | 2006 Incentive Plan |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Common stock, additional authorized shares |  |  |  |  |  | 2,723,000

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