Income Taxes (Deferred Taxes) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Components of Deferred Tax Assets and Liabilities [Abstract]
Deferred tax assets, plant lease turnover and other exit costs	$ 15.6	$ 18.9
Deferred tax assets, employee benefit costs	191.7	135.6
Deferred tax assets, inventory		15.1
Deferred tax assets, property plant and equipment	75.0	18.9
Deferred tax assets, tax intangibles	0.9	1.7
Deferred tax assets, deferred costs for depleted uranium	56.8	49.4
Deferred tax assets, net operating loss and credit carryforwards	22.3	1.6
Deferred tax assets, accrued expenses	7.7	9.2
Deferred tax assets, other	7.0	5.4
Deferred tax assets, gross, total	377.0	255.8
Deferred tax asssets, valuation allowance	(370.6)	(1.5)
Deferred tax assets, net	6.4	254.3
Deferred tax liabilities, inventory	1.5
Deferred tax liabilities, prepaid expenses	1.1	1.2
Deferred tax liabilities, dividends on preferred stock	3.8	1.1
Deferred tax liabilities	6.4	2.3
Deferred tax assets and liabilities, net		$ 252.0

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The sum of the tax effects as of the balance sheet date of the amounts of all future tax deductions arising from temporary differences between tax basis and generally accepted accounting principles basis recognition of assets, liabilities, revenues and expenses, which can only be deducted for tax purposes when permitted under enacted tax laws ( before the valuation allowance, if any, to reduce such sum amount to net realizable value). Includes any tax benefit realized in deferred tax assets for significant impacts of tax planning strategies.