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Label Element Value
Prospectus: rr_ProspectusTable  
Document Type dei_DocumentType 497
Document Period End Date dei_DocumentPeriodEndDate Jul. 31, 2018
Entity Registrant Name dei_EntityRegistrantName AMERICAN CENTURY MUTUAL FUNDS, INC.
Central Index Key dei_EntityCentralIndexKey 0000100334
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Jul. 31, 2018
Document Effective Date dei_DocumentEffectiveDate Mar. 01, 2018
Prospectus Date rr_ProspectusDate Mar. 01, 2018
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND  
Prospectus: rr_ProspectusTable  
Supplement Text Block [Text Block] ck0000100334_SupplementTextBlockTextBlock
American Century Mutual Funds, Inc.
Prospectus Supplement
Sustainable Equity Fund
newaci_logoblk.jpg
Supplement dated August 1, 2018 n Prospectus dated March 1, 2018

The following replaces the Annual Fund Operating Expenses table on page 2 of the prospectus.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | INVESTOR CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.84% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.85%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 87
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 272
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 472
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,049
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | I CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.64% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.65%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 67
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 208
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 363
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 811
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | A CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.49% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 51
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 161
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 629
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | C CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.84% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.10%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 681
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 905
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,147
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,838
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | R CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.84% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.85%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 188
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 583
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,001
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,167
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | R5 CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.84% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.50%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.35%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 138
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 428
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 740
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,623
American Century Mutual Funds, Inc | SUSTAINABLE EQUITY FUND | Y CLASS  
Prospectus: rr_ProspectusTable  
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.64% [1]
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.01%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.65%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 67
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 208
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 363
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 811
[1] The management fee has been restated to reflect the decrease in the management fee schedule effective August 1, 2018.