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)

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.

No definition available.