Quarterly report pursuant to Section 13 or 15(d)

Income Taxes (Detail)

Income Taxes (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Jun. 30, 2012
Net deferred tax assets   $ 314.7
Net financial statement tax expense 60.5  
Financial statement impact of which was an income tax benefit $ 26.9  
Scenario, Forecast [Member] | Iron Ore And Coal Mining Profits [Member]
Positive impact on the estimated annual effective tax rate   20.70%