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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FRANKLIN CUSTODIAN FUNDS
Prospectus Date rr_ProspectusDate Feb. 01, 2018
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary
Franklin Custodian Funds-20 | Franklin Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock To maximize income while maintaining prospects for capital appreciation.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

These tables describe the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts in Class A and Class M if you and your family invest, or agree to invest in the future, at least $100,000 in Franklin Templeton funds. More information about these and other discounts is available from your financial professional and under “Your Account” on page 111 in the Fund's Prospectus and under “Buying and Selling Shares” on page 77 of the Fund’s Statement of Additional Information. In addition, more information about sales charge discounts and waivers for purchases of shares through specific financial intermediaries is set forth in Appendix A – "Intermediary Sales Charge Discounts and Waivers" to the Fund’s prospectus.

Please note that the tables and examples below do not reflect any transaction fees that may be charged by financial intermediaries, or commissions that a shareholder may be required to pay directly to its financial intermediary when buying or selling Class R6 or Advisor Class shares.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder Fees (fees paid directly from your investment)
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)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual Fund operating expenses or in the Example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 34.12% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 34.12%
Expense Breakpoint Discounts [Text] rr_ExpenseBreakpointDiscounts You may qualify for sales charge discounts in Class A and Class M if you and your family invest, or agree to invest in the future, at least $100,000 in Franklin Templeton funds.
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 100,000
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of the period. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same. The Example reflects adjustments made to the Fund's operating expenses due to the fee waivers and/or expense reimbursements by management for the 1 Year numbers only. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption, By Year, Caption [Text] rr_ExpenseExampleNoRedemptionByYearCaption If you do not sell your shares:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

Under normal market conditions, the Fund invests in a diversified portfolio of debt and equity securities. The Fund may shift its investments from one asset class to another based on the investment manager's analysis of the best opportunities for the Fund’s portfolio in a given market. The equity securities in which the Fund invests consist primarily of common stocks. Debt securities include all varieties of fixed, floating and variable rate instruments, including secured and unsecured bonds, bonds convertible into common stock, senior floating rate and term loans, mortgage-backed securities and other asset-backed securities, debentures, and shorter term instruments. The Fund seeks income by selecting investments such as corporate, foreign and U.S. Treasury bonds, as well as stocks with dividend yields the investment manager believes are attractive. The Fund may invest up to 100% of its total assets in debt securities that are rated below investment grade (also known as "junk bonds"), including a portion in defaulted securities. The Fund maintains the flexibility to invest in securities of companies from a variety of sectors, but from time to time, based on economic conditions, the Fund may have significant investments in particular sectors. The Fund may also invest up to 25% of its assets in foreign securities, either directly or through depositary receipts.

The Fund may invest up to 15% of its net assets in equity-linked notes, which are hybrid derivative-type instruments that are specially designed to combine the characteristics of one or more reference securities (usually a single stock, a stock index or a basket of stocks (underlying securities)) and a related equity derivative, such as a put or call option, in a single note form.

The Fund's investment manager searches for undervalued or out-of-favor securities it believes offer opportunities for income today and significant growth tomorrow. It generally performs independent analysis of the debt securities being considered for the Fund's portfolio, rather than relying principally on the ratings assigned by rating organizations. In analyzing both debt and equity securities, the investment manager considers a variety of factors, including:

a security's relative value based on such factors as anticipated cash flow, interest or dividend coverage, asset coverage, and earnings prospects;

the experience and strength of the company's management;

the company's changing financial condition and market recognition of the change;

the company's sensitivity to changes in interest rates and business conditions; and

the company's debt maturity schedules and borrowing requirements.

When choosing equity investments for the Fund, the investment manager applies a "bottom-up," value oriented, long-term approach, focusing on the market price of a company's securities relative to the investment manager's evaluation of the company's long-term earnings, asset value and cash flow potential. The investment manager also considers a company's price/earnings ratio, profit margins and liquidation value.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

You could lose money by investing in the Fund. Mutual fund shares are not deposits or obligations of, or guaranteed or endorsed by, any bank, and are not insured by the Federal Deposit Insurance Corporation, the Federal Reserve Board, or any other agency of the U.S. government.

Market

The market values of securities or other investments owned by the Fund will go up or down, sometimes rapidly or unpredictably. The market value of a security or other investment may be reduced by market activity or other results of supply and demand unrelated to the issuer. This is a basic risk associated with all investments. When there are more sellers than buyers, prices tend to fall. Likewise, when there are more buyers than sellers, prices tend to rise.

Stock prices tend to go up and down more dramatically than those of debt securities. A slower-growth or recessionary economic environment could have an adverse effect on the prices of the various stocks held by the Fund.

High-Yield Debt Securities

Issuers of lower-rated or “high-yield” debt securities (also known as “junk bonds”) are not as strong financially as those issuing higher credit quality debt securities. High-yield debt securities are generally considered predominantly speculative by the applicable rating agencies as their issuers are more likely to encounter financial difficulties and are more vulnerable to changes in the relevant economy, such as a recession or a sustained period of rising interest rates, that could affect their ability to make interest and principal payments when due. The prices of high-yield debt securities generally fluctuate more than those of higher credit quality. High-yield debt securities are generally more illiquid (harder to sell) and harder to value.

Interest Rate

When interest rates rise, debt security prices generally fall. The opposite is also generally true: debt security prices rise when interest rates fall. Interest rate changes are influenced by a number of factors, including government policy, monetary policy, inflation expectations, perceptions of risk, and supply and demand of bonds. In general, securities with longer maturities or durations are more sensitive to interest rate changes.

Credit

An issuer of debt securities may fail to make interest payments or repay principal when due, in whole or in part. Changes in an issuer's financial strength or in a security's credit rating may affect a security's value.

Foreign Securities

Investing in foreign securities typically involves more risks than investing in U.S. securities, including risks related to currency exchange rates and policies, country or government specific issues, less favorable trading practices or regulation and greater price volatility. Certain of these risks also may apply to securities of U.S. companies with significant foreign operations.

Income

Because the Fund can only distribute what it earns, the Fund's distributions to shareholders may decline when prevailing interest rates fall or when the Fund experiences defaults on debt securities it holds.

Prepayment

Prepayment risk occurs when a debt security can be repaid in whole or in part prior to the security's maturity and the Fund must reinvest the proceeds it receives, during periods of declining interest rates, in securities that pay a lower rate of interest. Also, if a security has been purchased at a premium, the value of the premium would be lost in the event of prepayment. Prepayments generally increase when interest rates fall.

Focus

To the extent that the Fund focuses on particular countries, regions, industries, sectors or types of investment from time to time, the Fund may be subject to greater risks of adverse developments in such areas of focus than a fund that invests in a wider variety of countries, regions, industries, sectors or investments.

Equity-Linked Notes (ELNs)

ELNs may not perform as expected and could cause the Fund to realize significant losses including its entire principal investment. Other risks include counterparty risk, liquidity risk and imperfect correlation between ELNs and the underlying securities.

Management

The Fund is subject to management risk because it is an actively managed investment portfolio. The Fund's investment manager applies investment techniques and risk analyses in making investment decisions for the Fund, but there can be no guarantee that these decisions will produce the desired results.

Value Style Investing

A value stock may not increase in price as anticipated by the investment manager if other investors fail to recognize the company's value and bid up the price, the markets favor faster-growing companies, or the factors that the investment manager believes will increase the price of the security do not occur or do not have the anticipated effect.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund. The bar chart shows changes in the Fund's performance from year to year for Class A shares. The table shows how the Fund's average annual returns for 1 year, 5 years, 10 years or since inception, as applicable, compared with those of a broad measure of market performance. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. You can obtain updated performance information at franklintempleton.com or by calling (800) DIAL BEN/342-5236.

The secondary indices in the table below show how the Fund's performance compares to a group of securities that align with a portion of the Fund's portfolio and to the returns of an index of funds with similar investment objectives as the Fund, respectively.

Sales charges are not reflected in the bar chart, and if those charges were included, returns would be less than those shown.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The following bar chart and table provide some indication of the risks of investing in the Fund. The bar chart shows changes in the Fund's performance from year to year for Class A shares. The table shows how the Fund's average annual returns for 1 year, 5 years, 10 years or since inception, as applicable, compared with those of a broad measure of market performance.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone (800) DIAL BEN/342-5236
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress franklintempleton.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Class A Annual Total Returns
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads Sales charges are not reflected in the bar chart, and if those charges were included, returns would be less than those shown.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter:Q2'0918.79%
Worst Quarter:Q3'08-15.45%
Performance Table Heading rr_PerformanceTableHeading

Average Annual Total Returns
(figures reflect sales charges)

For the periods ended December 31, 2017
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

Performance information for Class M is not available because the share class is new.

No one index is representative of the Fund's portfolio.

The after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor's tax situation and may differ from those shown. After-tax returns are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown only for Class A and after-tax returns for other classes will vary.
Franklin Custodian Funds-20 | Franklin Income Fund | Class A  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumCumulativeSalesChargeOverOfferingPrice 4.25%
Maximum Deferred Sales Charge (Load) (as percentage of the lower of original purchase price or sale proceeds) rr_MaximumDeferredSalesChargeOverOfferingPrice none [1]
Management fees rr_ManagementFeesOverAssets 0.37%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.15%
Other expenses rr_OtherExpensesOverAssets 0.10% [2]
Acquired fund fees and expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total annual Fund operating expenses rr_ExpensesOverAssets 0.63% [3]
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [4]
Total annual Fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.62% [3],[4]
1 year rr_ExpenseExampleYear01 $ 486
3 years rr_ExpenseExampleYear03 617
5 years rr_ExpenseExampleYear05 760
10 years rr_ExpenseExampleYear10 $ 1,177
2008 rr_AnnualReturn2008 (30.51%)
2009 rr_AnnualReturn2009 35.01%
2010 rr_AnnualReturn2010 12.91%
2011 rr_AnnualReturn2011 2.83%
2012 rr_AnnualReturn2012 13.68%
2013 rr_AnnualReturn2013 14.23%
2014 rr_AnnualReturn2014 4.12%
2015 rr_AnnualReturn2015 (7.81%)
2016 rr_AnnualReturn2016 16.29%
2017 rr_AnnualReturn2017 8.41%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 18.79%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (15.45%)
Franklin Custodian Funds-20 | Franklin Income Fund | Class M  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumCumulativeSalesChargeOverOfferingPrice 4.25% [5]
Maximum Deferred Sales Charge (Load) (as percentage of the lower of original purchase price or sale proceeds) rr_MaximumDeferredSalesChargeOverOfferingPrice none [1],[5]
Management fees rr_ManagementFeesOverAssets 0.37%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.10% [2]
Acquired fund fees and expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total annual Fund operating expenses rr_ExpensesOverAssets 0.73% [3]
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [4]
Total annual Fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.72% [3],[4]
1 year rr_ExpenseExampleYear01 $ 495
3 years rr_ExpenseExampleYear03 647
5 years rr_ExpenseExampleYear05 813
10 years rr_ExpenseExampleYear10 $ 1,265
Franklin Custodian Funds-20 | Franklin Income Fund | Class C  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumCumulativeSalesChargeOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of the lower of original purchase price or sale proceeds) rr_MaximumDeferredSalesChargeOverOfferingPrice 1.00%
Management fees rr_ManagementFeesOverAssets 0.37%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.65%
Other expenses rr_OtherExpensesOverAssets 0.10% [2]
Acquired fund fees and expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total annual Fund operating expenses rr_ExpensesOverAssets 1.13% [3]
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [4]
Total annual Fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 1.12% [3],[4]
1 year rr_ExpenseExampleYear01 $ 214
3 years rr_ExpenseExampleYear03 358
5 years rr_ExpenseExampleYear05 621
10 years rr_ExpenseExampleYear10 1,374
1 Year rr_ExpenseExampleNoRedemptionYear01 114
3 Years rr_ExpenseExampleNoRedemptionYear03 358
5 Years rr_ExpenseExampleNoRedemptionYear05 621
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,374
Franklin Custodian Funds-20 | Franklin Income Fund | Class R  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumCumulativeSalesChargeOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of the lower of original purchase price or sale proceeds) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management fees rr_ManagementFeesOverAssets 0.37%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.50%
Other expenses rr_OtherExpensesOverAssets 0.10% [2]
Acquired fund fees and expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total annual Fund operating expenses rr_ExpensesOverAssets 0.98% [3]
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [4]
Total annual Fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.97% [3],[4]
1 year rr_ExpenseExampleYear01 $ 99
3 years rr_ExpenseExampleYear03 311
5 years rr_ExpenseExampleYear05 541
10 years rr_ExpenseExampleYear10 $ 1,201
Franklin Custodian Funds-20 | Franklin Income Fund | Class R6  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumCumulativeSalesChargeOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of the lower of original purchase price or sale proceeds) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management fees rr_ManagementFeesOverAssets 0.37%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.04% [2]
Acquired fund fees and expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total annual Fund operating expenses rr_ExpensesOverAssets 0.42% [3]
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.02%) [4]
Total annual Fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.40% [3],[4]
1 year rr_ExpenseExampleYear01 $ 41
3 years rr_ExpenseExampleYear03 133
5 years rr_ExpenseExampleYear05 233
10 years rr_ExpenseExampleYear10 $ 528
Franklin Custodian Funds-20 | Franklin Income Fund | Advisor Class  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumCumulativeSalesChargeOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of the lower of original purchase price or sale proceeds) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management fees rr_ManagementFeesOverAssets 0.37%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.10% [2]
Acquired fund fees and expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total annual Fund operating expenses rr_ExpensesOverAssets 0.48% [3]
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [4]
Total annual Fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.47% [3],[4]
1 year rr_ExpenseExampleYear01 $ 48
3 years rr_ExpenseExampleYear03 153
5 years rr_ExpenseExampleYear05 268
10 years rr_ExpenseExampleYear10 $ 603
Franklin Custodian Funds-20 | Franklin Income Fund | Return Before Taxes | Class A  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Franklin Income Fund
Label rr_AverageAnnualReturnLabel Return Before Taxes
Past 1 year rr_AverageAnnualReturnYear01 3.91%
Past 5 years rr_AverageAnnualReturnYear05 5.76%
Past 10 years rr_AverageAnnualReturnYear10 5.08%
Franklin Custodian Funds-20 | Franklin Income Fund | Return Before Taxes | Class C  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 7.22%
Past 5 years rr_AverageAnnualReturnYear05 6.17%
Past 10 years rr_AverageAnnualReturnYear10 5.02%
Franklin Custodian Funds-20 | Franklin Income Fund | Return Before Taxes | Class R  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 8.18%
Past 5 years rr_AverageAnnualReturnYear05 6.33%
Past 10 years rr_AverageAnnualReturnYear10 5.18%
Franklin Custodian Funds-20 | Franklin Income Fund | Return Before Taxes | Class R6  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 8.67%
Past 10 years rr_AverageAnnualReturnYear10
Since Inception rr_AverageAnnualReturnSinceInception 5.80% [6]
Franklin Custodian Funds-20 | Franklin Income Fund | Return Before Taxes | Advisor Class  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 8.62%
Past 5 years rr_AverageAnnualReturnYear05 6.80%
Past 10 years rr_AverageAnnualReturnYear10 5.71%
Franklin Custodian Funds-20 | Franklin Income Fund | After Taxes on Distributions | Class A  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Franklin Income Fund
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
Past 1 year rr_AverageAnnualReturnYear01 2.00%
Past 5 years rr_AverageAnnualReturnYear05 3.78%
Past 10 years rr_AverageAnnualReturnYear10 2.96%
Franklin Custodian Funds-20 | Franklin Income Fund | After Taxes on Distributions and Sales | Class A  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Franklin Income Fund
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year rr_AverageAnnualReturnYear01 2.52%
Past 5 years rr_AverageAnnualReturnYear05 3.69%
Past 10 years rr_AverageAnnualReturnYear10 3.11%
Franklin Custodian Funds-20 | Franklin Income Fund | S&P 500® Index (index reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 21.83%
Past 5 years rr_AverageAnnualReturnYear05 15.79%
Past 10 years rr_AverageAnnualReturnYear10 8.50%
Franklin Custodian Funds-20 | Franklin Income Fund | Bloomberg Barclays U.S. Aggregate Index (index reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 3.54%
Past 5 years rr_AverageAnnualReturnYear05 2.10%
Past 10 years rr_AverageAnnualReturnYear10 4.00%
Franklin Custodian Funds-20 | Franklin Income Fund | Lipper Mixed-Asset Target Allocation Moderate Funds Classification Average (index reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 12.75%
Past 5 years rr_AverageAnnualReturnYear05 7.29%
Past 10 years rr_AverageAnnualReturnYear10 4.95%
[1] There is a 0.75% contingent deferred sales charge that applies to investments of $1 million or more (see "Investments of $1 Million or More" under "Choosing a Share Class") and purchases by certain retirement plans without an initial sales charge on shares sold within 18 months of purchase.
[2] The Fund began offering Class M shares on January 25, 2018. Other expenses for Class M are based on estimated amounts for the current year.
[3] Total annual Fund operating expenses differ from the ratio of expenses to average net assets shown in the Financial Highlights, which reflect the operating expenses of the Fund and do not include acquired fund fees and expenses.
[4] The investment manager has contractually agreed in advance to reduce its fee as a result of the Fund’s investment in other Franklin Templeton funds (acquired fund) for the next 12-month period. In addition, the transfer agent has contractually agreed to waive or limit its transfer agency fees for Class R6 shares of the Fund so that transfer agency fees for that Class do not exceed 0.01% until January 31, 2019. Contractual fee waiver and/or expense reimbursement agreements may not be changed or terminated during the time periods set forth above.
[5] The Fund began offering Class M shares on January 25, 2018.
[6] Since inception May 1, 2013.