Income Taxes (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Mineral Resources Rent Tax [Member]
Net deferred tax assets	$ 314.7
Positive impact on the estimated annual effective tax rate	49.10%
Impact of Foreign Currency Elections [Member]
Net financial statement tax expense	60.5
FIN 48 [Member]
Financial statement impact of which was an income tax benefit	$ 26.9