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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Document Type dei_DocumentType Other
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2011
Registrant Name dei_EntityRegistrantName COLUMBIA ACORN TRUST
Central Index Key dei_EntityCentralIndexKey 0000002110
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Jan. 24, 2012
Document Effective Date dei_DocumentEffectiveDate Jan. 24, 2012
Prospectus Date rr_ProspectusDate Jan. 24, 2012
(Columbia Acorn Select)
  
Risk/Return: rr_RiskReturnAbstract  
Supplement [Text Block] columbia_SupplementTextBlock
COLUMBIA ACORN TRUST
Columbia Acorn Select℠
(the "Fund")

Supplement dated January 24, 2012 to the Prospectuses dated May 1, 2011, as supplemented,
and the Statement of Additional Information dated May 1, 2011

Effective January 31, 2012, the Fund will no longer be classified as a non-diversified investment company within the meaning of Section 5(b) of the Investment Company Act of 1940.

Effective January 31, 2012, the Fund's prospectuses dated May 1, 2011, as supplemented, are further supplemented as follows:
Strategy Narrative Supplement [Text Block] columbia_SupplementTextBlock_02

The last sentence of the fourth paragraph under the heading Principal Investment Strategies is replaced in its entirety with the following:

Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

The Fund was previously registered as a non-diversified investment company. The Fund has changed its classification from non-diversified to diversified. The Fund will not be able to become non-diversified unless it seeks and obtains the approval of shareholders.
Risk Narrative Supplement [Text Block] columbia_SupplementTextBlock_01

The last bullet point under the heading Principal Risks is deleted in its entirety.