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Income Taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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Deferred tax assets: |  |
Accrued postretirement benefit cost | $ 29.5 | $ 38.9
Net operating losses and tax credits | 453.1 | 430.3
Disallowed interest expense | 17.8 | 6.9
Foreign basis differences | 0.0 | 16.2
Derivative and hedging contracts | 18.6 | 0.5
Other | 13.9 | 12.3
Total deferred tax assets | 532.9 | 505.1
Valuation allowance | (485.8) | (499.4)
Net deferred tax assets | 47.1 | 5.7
Deferred tax liabilities: |  |
Tax over financial statement depreciation | (86.2) | (95.0)
Foreign basis differences | (19.6) | 0.0
Total deferred tax liabilities | (105.8) | (95.0)
Net deferred tax liability | $ (58.7) | $ (89.3)

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- Definition The amount as of the balance sheet date of the estimated future tax effects attributable to the difference between the tax basis and the basis in accordance with generally accepted accounting principles of foreign basis differences which will decrease future taxable income when such basis difference reverses. + References No definition available. + Details
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