Write-downs, Reserves, and Project Opening Costs, net of Recoveries - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Divestitures and abandonments charges		$ 28.1	$ 4.2	$ 42.7	$ 3.9
Gain on property	11.0	11.0		11.0
Mississippi [Member]
Divestitures and abandonments charges		$ 20.2		$ 24.7