Write-downs, Reserves, and Project Opening Costs, net of Recoveries - Components of Write-Downs, Reserves, and Project Opening Costs, Net of Recoveries (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
Write-downs and reserves, net of recoveries:
Divestitures and abandonments		$ 28.1	$ 4.2	$ 42.7	$ 3.9
Remediation costs		6.4	1.7	12.4	9.4
Efficiency projects		1.8	10.6	9.6	36.1
Gain on Thistledown contribution	(11.0)	(11.0)		(11.0)
Other		4.5	(4.0)	5.0	6.4
Total write-downs and reserves, net of recoveries		29.8	12.5	58.7	55.8
Project opening costs		2.8		4.7	4.2
Total write-downs, reserves, and project opening costs, net of recoveries		$ 32.6	$ 12.5	$ 63.4	$ 60.0