CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current assets:
Cash and cash equivalents	$ 17.7	$ 16.9	$ 23.7	$ 9.3	$ 17.9	$ 21.2	$ 26.8	$ 27.7
Accounts receivable, net	219.9		204.8				190.1
Inventories	160.8		144.8				105.6
Prepaid expenses	19.4		15.7				13.0
Deferred income taxes	6.8		5.9				5.5
Income tax receivable	15.1		6.9				8.5
Other	2.6		6.1				6.2
Total current assets	442.3		407.9				355.7
Property, plant, equipment, and mineral rights and reserves:
Land	12.9		13.9				10.9
Mineral rights	49.9		52.5				65.0
Depreciable assets	535.6		482.6				452.3
Gross property, plant and equipment	598.4		549.0				528.2
Less: accumulated depreciation and depletion	303.4		275.5				256.7
Net property, plant and equipment	295.0		273.5				271.5
Goodwill	69.9		69.5				70.9
Intangible assets, net	34.8		36.6				41.9
Investment in and advances to affiliates and joint ventures	28.0		26.4				20.9
Available-for-sale securities	7.9		3.8				14.2
Deferred income taxes	9.3		8.5				9.2
Other assets	25.6		24.4				23.5
Total noncurrent assets	470.5		442.7				452.1
Total Assets	912.8		850.6				807.8
Current liabilities:
Accounts payable	52.7		56.5				51.4
Accrued income taxes	18.2		2.4				5.1
Accrued liabilities	66.9		57.9				55.2
Total current liabilities	137.8		116.8				111.7
Noncurrent liabilities:
Long-term debt	244.1		260.7				235.7
Pension liabilities	35.3		34.8				21.3
Deferred compensation	10.3		8.9				9.0
Deferred income taxes	13.0		13.5				16.6
Other long-term liabilities	21.7		19.2				18.4
Total noncurrent liabilities	324.4		337.1				301.0
Shareholders' Equity:
Common stock	0.3		0.3				0.3
Additional paid in capital	100.8		94.3				95.1
Retained earnings	352.1		316.4				279.6
Accumulated other comprehensive income (loss)	(5.7)		(15.0)				28.7
Less: Treasury stock	(0.7)		(3.4)				(8.9)
Total AMCOL shareholders' equity	446.8		392.6				394.8
Noncontrolling interest	3.8		4.1				0.3
Total equity	450.6		396.7	401.4			395.1
Total Liabilities and Shareholders' Equity	$ 912.8		$ 850.6				$ 807.8