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Tax Effect of Temporary Differences and Tax Attributes Representing Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Deferred tax assets: |  |
Net operating loss carryforwards | $ 370,875 | $ 375,308
Tax credits | 4,918 | 4,138
Expense associated with stock-based compensation | 11,252 | 10,561
Workers' compensation allowance | 17,177 | 19,536
Other deferred tax asset | 24,735 | 21,698
Total deferred tax assets, gross | 428,957 | 431,241
Total deferred tax assets, net | 409,824 | 414,010
Valuation allowance | (19,133) | (17,231)
Deferred tax liabilities: |  |
Property and equipment basis difference | (475,025) | (607,785)
Other | (12,475) | (9,184)
Total deferred tax liabilities | (487,500) | (616,969)
Net deferred tax liability | $ (77,676) | $ (202,959)

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