Noncontrolling Interest (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Equity attributable to AMCOL's shareholders
Net income attributable to AMCOL shareholders	$ 11.0	$ 19.0	$ 21.7	$ 13.4	$ 12.8	$ 19.5	$ 13.1	$ 13.1	$ 65.1	$ 58.5	$ 29.2
Transfers from noncontrolling interest:
Decrease in additional paid-in capital for purchase of the remaining noncontrolling interest in South Africa (Proprietary) Limited										(5.7)	2.7
Decrease in additional paid-in capital for transfer of 26% interest in Batlhako										0
AMCOL Shareholders [Member]
Equity attributable to AMCOL's shareholders
Net income attributable to AMCOL shareholders									65.1	58.5
Transfers from noncontrolling interest:
Decrease in additional paid-in capital for purchase of the remaining noncontrolling interest in South Africa (Proprietary) Limited									0	(5.4)
Decrease in additional paid-in capital for transfer of 26% interest in Batlhako									0	(3.4)
Change from net income attributable to AMCOL shareholders and transfers from noncontrolling interest									$ 65.1	$ 49.7