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Risk/Return:rr_RiskReturnAbstract 
Document Typedei_DocumentTypeOther
Document Period End Datedei_DocumentPeriodEndDateSep. 30, 2011
Registrant Namedei_EntityRegistrantNameAMERICAN CENTURY MUTUAL FUNDS, INC.
Central Index Keydei_EntityCentralIndexKey0000100334
Amendment Flagdei_AmendmentFlagfalse
Document Creation Datedei_DocumentCreationDateSep. 30, 2011
Document Effective Datedei_DocumentEffectiveDateOct. 01, 2011
Prospectus Daterr_ProspectusDateMar. 01, 2011
NEW OPPORTUNITIES FUND
 
Risk/Return:rr_RiskReturnAbstract 
Shareholder Fees Caption [Text]rr_ShareholderFeesCaptionShareholder Fees (fees paid directly from your investment)
Supplement [Text Block]acmf_SupplementTextBlock
American Century Mutual Funds, Inc.
Prospectus Supplement
 
New Opportunities Fund
 
 
Supplement dated October 1, 2011 ■ Prospectus dated March 1, 2011

Effective November 14, 2011, the fund’s redemption fee applicable to Investor, Institutional and R Class shares applies only to shares held less than 60 days.

The following replaces the redemption/exchange fee language in the Shareholder Fees table on page 2 of the prospectus:

Shareholder Fees (fees paid directly from your investment)
   
Investor
Institutional
A
C
R
Redemption/Exchange Fee (as a percentage of amount redeemed/exchanged for shares held less than 180 days prior to November 14, 2011, or less than 60 days on or after November 14, 2011)
 
2.00%
2.00%
None
None
2.00%

The following replaces the first sentence of the Redemptions section on page 17 of the prospectus:

Investor, Institutional and R Class shares held for less than 180 days are subject to a redemption fee of 2.00%. Effective November 14, 2011, Investor, Institutional and R Class shares held for less than 60 days are subject to a redemption fee of 2.00%.
NEW OPPORTUNITIES FUND | INVESTOR CLASS
 
Risk/Return:rr_RiskReturnAbstract 
Redemption/Exchange Fee (as a percentage of amount redeemed/exchanged for shares held less than 180 days prior to November 14, 2011, or less than 60 days on or after November 14, 2011)rr_ExchangeFeeOverRedemption2.00%
NEW OPPORTUNITIES FUND | INSTITUTIONAL CLASS
 
Risk/Return:rr_RiskReturnAbstract 
Redemption/Exchange Fee (as a percentage of amount redeemed/exchanged for shares held less than 180 days prior to November 14, 2011, or less than 60 days on or after November 14, 2011)rr_ExchangeFeeOverRedemption2.00%
NEW OPPORTUNITIES FUND | A CLASS
 
Risk/Return:rr_RiskReturnAbstract 
Redemption/Exchange Fee (as a percentage of amount redeemed/exchanged for shares held less than 180 days prior to November 14, 2011, or less than 60 days on or after November 14, 2011)rr_ExchangeFeeOverRedemption none
NEW OPPORTUNITIES FUND | C CLASS
 
Risk/Return:rr_RiskReturnAbstract 
Redemption/Exchange Fee (as a percentage of amount redeemed/exchanged for shares held less than 180 days prior to November 14, 2011, or less than 60 days on or after November 14, 2011)rr_ExchangeFeeOverRedemption none
NEW OPPORTUNITIES FUND | R CLASS
 
Risk/Return:rr_RiskReturnAbstract 
Redemption/Exchange Fee (as a percentage of amount redeemed/exchanged for shares held less than 180 days prior to November 14, 2011, or less than 60 days on or after November 14, 2011)rr_ExchangeFeeOverRedemption2.00%