Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

COLUMBIA ACORN TRUST

Prospectus Date

rr_ProspectusDate

May 01, 2017

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn^® Fund

Columbia Acorn International^®

Columbia Acorn USA^®

Columbia Acorn International Select^SM

Columbia Acorn Select^SM

Columbia Thermostat Fund^SM

Columbia Acorn Emerging Markets Fund^SM

Columbia Acorn European Fund^SM

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:                                                                              

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn^® Fund

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn International

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn International^®

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn USA

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn USA^®

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn International Select

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn International Select^SM

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn Select

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn Select^SM

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Thermostat Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Thermostat Fund^SM

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn Emerging Markets Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn Emerging Markets Fund^SM

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)

Columbia Acorn European Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated November 1, 2017

to the Prospectuses and Statement of Additional Information (SAI) dated May 1, 2017,

as supplemented, for the following Funds:

Funds

COLUMBIA ACORN TRUST

Columbia Acorn European Fund^SM

Effective immediately, each Fund’s Prospectus and SAI are supplemented as follows:

Class R4, Class R5, Class Y and Class Z shares are renamed and all references to such share classes are hereby replaced with the following new share class names:

Former Share Class Name	New Share Class Name (and abbreviation)
(prior to November 1, 2017)	(as of November 1, 2017)
Class R4	Advisor Class (Class Adv)
Class R5	Institutional 2 Class (Class Inst2)
Class Y	Institutional 3 Class (Class Inst3)
Class Z	Institutional Class (Class Inst)