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Income Taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Deferred tax assets: |  |
Accrued postretirement benefit cost | $ 38.9 | $ 39.3
Accrued liabilities | 7.2 | 10.8
Share-based compensation | 0.7 | 0.9
Net operating losses and tax credits | 430.3 | 432.8
Foreign basis differences | 16.2 | 15.0
Ravenswood retiree legal settlement | 1.7 | 1.9
Other | 10.1 | 2.8
Total deferred tax assets | 505.1 | 503.5
Valuation allowance | (499.4) | (492.4)
Net deferred tax assets | 5.7 | 11.1
Deferred tax liabilities: |  |
Derivatives | 0.0 | (3.5)
Tax over financial statement depreciation | (95.0) | (102.4)
Total deferred tax liabilities | (95.0) | (105.9)
Net deferred tax liability | $ (89.3) | $ (94.8)

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