CORRECTION OF ERRORS IN PREVIOUSLY REPORTED FINANCIAL STATEMENTS (DETAILS 2) (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
Previously reported [Member]
Current assets:
Cash and cash equivalents		16.9	23.7	9.3	17.9	21.2	27.3	27.7
Accounts receivable, net			206.8				194.0
Inventories			146.6				107.5
Prepaid expenses			15.7				12.6
Deferred income taxes			5.9				5.5
Income tax receivable			6.9				8.5
Other			6.7				6.2
Total current assets			412.3				361.6
Property, plant, equipment, and mineral rights and reserves:
Land			13.9				11.6
Mineral rights			41.9				51.4
Depreciable assets			482.3				454.4
Gross property, plant and equipment			538.1				517.4
Less: accumulated depreciation and depletion			275.5				256.9
Net property, plant and equipment			262.6				260.5
Goodwill			69.5				70.9
Intangible assets, net			36.6				42.6
Investment in and advances to affiliates and joint ventures			26.4				19.0
Available-for-sale securities			3.8				14.2
Deferred income taxes			7.8				7.6
Other assets			23.7				22.7
Total noncurrent assets			430.4				437.5
Total Assets			842.7				799.1
Current liabilities:
Accounts payable			56.4				53.2
Accrued income taxes			2.7				4.1
Accrued liabilities			59.0				55.2
Total current liabilities			118.1				112.5
Noncurrent liabilities:
Long-term debt			260.7				236.2
Pension liabilities			34.8				21.3
Deferred compensation			8.9				8.7
Deferred income taxes			0.8				0.4
Other long-term liabilities			19.2				19.6
Total noncurrent liabilities			324.4				286.2
Shareholders' Equity:
Common stock			0.3				0.3
Additional paid in capital			94.5				95.1
Retained earnings			319.5				283.2
Accumulated other comprehensive income (loss)			(14.9)				28.9
Less: Treasury stock			(3.4)				(8.9)
Total AMCOL shareholders' equity			396.0				398.6
Noncontrolling interest			4.2				1.8
Total equity			400.2				400.4
Total Liabilities and Shareholders' Equity			842.7				799.1
Adjustments [Member]
Current assets:
Cash and cash equivalents		0	0	0	0	0	(0.5)	0
Accounts receivable, net			(2.0)				(3.9)
Inventories			(1.8)				(1.9)
Prepaid expenses							0.4
Other			(0.6)
Total current assets			(4.4)				(5.9)
Property, plant, equipment, and mineral rights and reserves:
Land							(0.7)
Mineral rights			10.6				13.6
Depreciable assets			0.3				(2.1)
Gross property, plant and equipment			10.9				10.8
Less: accumulated depreciation and depletion							(0.2)
Net property, plant and equipment			10.9				11.0
Intangible assets, net							(0.7)
Investment in and advances to affiliates and joint ventures							1.9
Deferred income taxes			0.7				1.6
Other assets			0.7				0.8
Total noncurrent assets			12.3				14.6
Total Assets			7.9				8.7
Current liabilities:
Accounts payable			0.1				(1.8)
Accrued income taxes			(0.3)				1.0
Accrued liabilities			(1.1)
Total current liabilities			(1.3)				(0.8)
Noncurrent liabilities:
Long-term debt							(0.5)
Deferred compensation							0.3
Deferred income taxes			12.7				16.2
Other long-term liabilities							(1.2)
Total noncurrent liabilities			12.7				14.8
Shareholders' Equity:
Additional paid in capital			(0.2)
Retained earnings			(3.1)				(3.6)
Accumulated other comprehensive income (loss)			(0.1)				(0.2)
Total AMCOL shareholders' equity			(3.4)				(3.8)
Noncontrolling interest			(0.1)				(1.5)
Total equity			(3.5)				(5.3)
Total Liabilities and Shareholders' Equity			7.9				8.7
Tax [Member] | Adjustments [Member]
Current assets:
Cash and cash equivalents			0				0
Total current assets			0				0
Property, plant, equipment, and mineral rights and reserves:
Gross property, plant and equipment			0				0
Net property, plant and equipment			0				0
Total noncurrent assets			0				0
Total Assets			0				0
Current liabilities:
Accounts payable			0
Accrued income taxes			0.1				1.2
Total current liabilities			0.1				1.2
Noncurrent liabilities:
Deferred income taxes							0.4
Total noncurrent liabilities			0				0.4
Shareholders' Equity:
Retained earnings			(0.1)				(1.6)
Total AMCOL shareholders' equity			(0.1)				(1.6)
Total equity			(0.1)				(1.6)
Total Liabilities and Shareholders' Equity			0				0
Other [Member] | Adjustments [Member]
Current assets:
Cash and cash equivalents			0				(0.5)
Accounts receivable, net			0.3				(2.4)
Inventories			(0.2)				(0.9)
Prepaid expenses			0				0.4
Deferred income taxes			0				0
Income tax receivable			0				0
Other			(0.6)				0
Total current assets			(0.5)				(3.4)
Property, plant, equipment, and mineral rights and reserves:
Land							(0.7)
Mineral rights			0				0
Depreciable assets			0.3				(2.1)
Gross property, plant and equipment			0.3				(2.8)
Less: accumulated depreciation and depletion			0				(0.2)
Net property, plant and equipment			0.3				(2.6)
Intangible assets, net			0				(0.7)
Investment in and advances to affiliates and joint ventures			0				1.9
Available-for-sale securities			0				0
Deferred income taxes			0				0
Other assets			0				0.3
Total noncurrent assets			0.3				(1.1)
Total Assets			(0.2)				(4.5)
Current liabilities:
Accounts payable			0.1				(1.1)
Accrued income taxes			(0.2)				(0.1)
Accrued liabilities			0				(0.1)
Total current liabilities			(0.1)				(1.3)
Noncurrent liabilities:
Long-term debt			0				(0.5)
Pension liabilities			0				0
Deferred compensation			0				0.3
Deferred income taxes			0.1				0
Other long-term liabilities			0				(1.2)
Total noncurrent liabilities			0.1				(1.4)
Shareholders' Equity:
Common stock			0				0
Additional paid in capital			0				0
Retained earnings			0.1				(0.4)
Accumulated other comprehensive income (loss)			(0.3)				(0.1)
Less: Treasury stock			0				0
Total AMCOL shareholders' equity			(0.2)				(0.5)
Noncontrolling interest			0				(1.3)
Total equity			(0.2)				(1.8)
Total Liabilities and Shareholders' Equity			(0.2)				(4.5)
Spain [Member] | Adjustments [Member]
Current assets:
Cash and cash equivalents			0				0
Accounts receivable, net			(1.6)				(0.4)
Inventories			(1.8)				(1.0)
Total current assets			(3.4)				(1.4)
Property, plant, equipment, and mineral rights and reserves:
Gross property, plant and equipment			0				0
Net property, plant and equipment			0				0
Total noncurrent assets			0				0
Total Assets			(3.4)				(1.4)
Current liabilities:
Accounts payable			0				0
Accrued income taxes			(0.3)				(0.1)
Accrued liabilities			(0.8)				0.1
Total current liabilities			(1.1)				0
Noncurrent liabilities:
Total noncurrent liabilities			0				0
Shareholders' Equity:
Retained earnings			(2.5)				(1.4)
Accumulated other comprehensive income (loss)			0.2
Total AMCOL shareholders' equity			(2.3)				(1.4)
Total equity			(2.3)				(1.4)
Total Liabilities and Shareholders' Equity			(3.4)				(1.4)
South Africa [Member] | Adjustments [Member]
Current assets:
Cash and cash equivalents			0				0
Inventories			0.2
Total current assets			0.2				0
Property, plant, equipment, and mineral rights and reserves:
Mineral rights			10.6				13.6
Gross property, plant and equipment			10.6				13.6
Net property, plant and equipment			10.6				13.6
Deferred income taxes			0.7				1.6
Total noncurrent assets			11.3				15.2
Total Assets			11.5				15.2
Current liabilities:
Accounts payable			0				0
Accrued income taxes			0.1
Total current liabilities			0.1				0
Noncurrent liabilities:
Deferred income taxes			12.6				15.8
Total noncurrent liabilities			12.6				15.8
Shareholders' Equity:
Additional paid in capital			(0.2)
Retained earnings			(0.9)				(0.3)
Accumulated other comprehensive income (loss)							(0.1)
Total AMCOL shareholders' equity			(1.1)				(0.4)
Noncontrolling interest			(0.1)				(0.2)
Total equity			(1.2)				(0.6)
Total Liabilities and Shareholders' Equity			11.5				15.2
Malaysia [Member] | Adjustments [Member]
Current assets:
Cash and cash equivalents			0				0
Accounts receivable, net			(0.7)				(1.1)
Total current assets			(0.7)				(1.1)
Property, plant, equipment, and mineral rights and reserves:
Gross property, plant and equipment			0				0
Net property, plant and equipment			0				0
Other assets			0.7				0.5
Total noncurrent assets			0.7				0.5
Total Assets			0				(0.6)
Current liabilities:
Accounts payable			0				0
Accrued income taxes							(0.2)
Accrued liabilities			(0.3)
Total current liabilities			(0.3)				(0.2)
Noncurrent liabilities:
Total noncurrent liabilities			0				0
Shareholders' Equity:
Retained earnings			0.3				(0.4)
Total AMCOL shareholders' equity			0.3				(0.4)
Total equity			0.3				(0.4)
Total Liabilities and Shareholders' Equity			0				(0.6)
Dong Ming [Member] | Adjustments [Member]
Current assets:
Cash and cash equivalents			0				0
Total current assets			0				0
Property, plant, equipment, and mineral rights and reserves:
Gross property, plant and equipment			0				0
Net property, plant and equipment			0				0
Total noncurrent assets			0				0
Total Assets			0				0
Current liabilities:
Accounts payable			0				(0.7)
Accrued income taxes							0.2
Total current liabilities			0				(0.5)
Noncurrent liabilities:
Total noncurrent liabilities			0				0
Shareholders' Equity:
Retained earnings			0				0.5
Total AMCOL shareholders' equity			0				0.5
Total equity			0				0.5
Total Liabilities and Shareholders' Equity			$ 0				$ 0