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%