Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

T. Rowe Price Mid-Cap Growth Fund, Inc.

Prospectus Date

rr_ProspectusDate

May 01, 2015

R Class Shares | T. Rowe Price Mid-Cap Growth Fund, Inc.

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

T. Rowe Price

Mid-Cap Growth Fund—R Class

SUMMARY

Objective [Heading]

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Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to provide long-term capital appreciation by investing in mid-cap stocks with potential for above-average earnings growth.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy and hold shares of the fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Fees and Expenses of the Fund’s R Class

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 26.6% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

26.60%

Expense Example [Heading]

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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 those periods. The example also assumes that your investment has a 5% return each year and that the fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies

Strategy Narrative [Text Block]

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The fund will normally invest at least 80% of its net assets (including any borrowings for investment purposes) in a diversified portfolio of common stocks of mid-cap companies whose earnings T. Rowe Price expects to grow at a faster rate than the average company. The fund defines mid-cap companies as those whose market capitalization (number of shares outstanding multiplied by share price) falls within the range of either the S&P MidCap 400 Index or the Russell Midcap Growth Index. As of December 31, 2014, the market capitalization ranges for the S&P MidCap 400 Index and the Russell Midcap Growth Index were approximately $842.8 million to $13.9 billion, and $275.2 million to $32.3 billion, respectively. The market capitalization of the companies in the fund’s portfolio and the S&P and Russell indices changes over time; the fund will not automatically sell or cease to purchase stock of a company it holds just because the company’s market capitalization grows or falls outside these ranges.

As “growth” investors, T. Rowe Price believes that when a company’s earnings grow faster than both inflation and the overall economy, the market will eventually reward it with a higher stock price.

In selecting investments, we generally favor companies with one or more of the following:

• proven products or services;

• a record of above-average earnings growth;

• demonstrated potential to sustain earnings growth;

• connection to an industry experiencing increasing demand; or

• stock prices that appear to undervalue their growth prospects.

In pursuing its investment objective, the fund has the discretion to deviate from its normal investment criteria. These situations might arise when the fund’s management believes a security could increase in value for a variety of reasons, including an extraordinary corporate event, a new product introduction or innovation, a favorable competitive development, or a change in management.

While most assets will typically be invested in U.S. common stocks, the fund may invest in foreign stocks in keeping with the fund’s objectives.

The fund may sell securities for a variety of reasons, such as to secure gains, limit losses, or redeploy assets into more promising opportunities.

Risk [Heading]

rr_RiskHeading

Principal Risks

Risk Narrative [Text Block]

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As with any mutual fund, there is no guarantee that the fund will achieve its objective. The fund's share price fluctuates, which means you could lose money by investing in the fund. The principal risks of investing in this fund are summarized as follows:

Active management risk The fund is subject to the risk that the investment adviser's judgments about the attractiveness, value, or potential appreciation of the fund's investments may prove to be incorrect. If the securities selected and strategies employed by the fund fail to produce the intended results, the fund could underperform other funds with similar objectives and investment strategies.

Risks of stock investing Stocks generally fluctuate in value more than bonds and may decline significantly over short time periods. There is a chance that stock prices overall will decline because stock markets tend to move in cycles, with periods of rising and falling prices. The value of a stock in which the fund invests may decline due to general weakness in the stock market or because of factors that affect a particular company or industry.

Market capitalization risk Investing primarily in issuers within the same market capitalization category carries the risk that the category may be out of favor due to current market conditions or investor sentiment. Because the fund invests primarily in securities issued by mid-cap companies, it is likely to be more volatile than a fund that focuses on securities issued by large companies. Medium-sized companies typically have less seasoned management, narrower product lines, and less capital reserves and liquidity than larger companies, and are therefore more sensitive to economic, market, and industry changes.

Investment style risk Different investment styles tend to shift in and out of favor depending on market conditions and investor sentiment. The fund's growth approach to investing could cause it to underperform other stock funds that employ a different investment style. Growth stocks tend to be more volatile than certain other types of stocks, and their prices may fluctuate more dramatically than the overall stock market. A stock with growth characteristics can have sharp price declines due to decreases in current or expected earnings and may lack dividends that can help cushion its share price in a declining market.

Foreign investing risk This is the risk that the fund's investments in foreign securities may be adversely affected by political, social, and economic conditions overseas, greater volatility, reduced liquidity, or decreases in foreign currency values relative to the U.S. dollar.

Risk Lose Money [Text]

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The fund's share price fluctuates, which means you could lose money by investing in the fund.

Bar Chart and Performance Table [Heading]

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Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The bar chart showing calendar year returns and the average annual total returns table indicate risk by illustrating how much returns can differ from one year to the next and how fund performance compares with that of a comparable market index. The fund’s past performance (before and after taxes) is not necessarily an indication of future performance.

The fund can also experience short-term performance swings, as shown by the best and worst calendar quarter returns during the years depicted.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The bar chart showing calendar year returns and the average annual total returns table indicate risk by illustrating how much returns can differ from one year to the next and how fund performance compares with that of a comparable market index.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-638-8790

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

troweprice.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 future performance.

Bar Chart [Heading]

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Mid-Cap Growth Fund–R Class
Calendar Year Returns

Bar Chart Closing [Text Block]

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	Quarter   Ended	Total Return
Best Quarter	9/30/09	18.95%
Worst Quarter	12/31/08	-25.37%

Performance Table Heading

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Average Annual Total Returns

Periods ended
December 31, 2014

Performance Table Uses Highest Federal Rate

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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.

Performance Table Not Relevant to Tax Deferred

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Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their fund shares through tax-deferred arrangements, such as a 401(k) account or individual retirement account.

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

In some cases, the figure shown under “returns after taxes on distributions and sale of fund shares” may be higher than the figure shown under “returns before taxes” because the calculations assume the investor received a tax deduction for any loss incurred on the sale of shares.

Performance Table Narrative

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In addition, the average annual total returns table shows hypothetical after-tax returns to suggest how taxes paid by a shareholder may influence returns. 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 shown are not relevant to investors who hold their fund shares through tax-deferred arrangements, such as a 401(k) account or individual retirement account. In some cases, the figure shown under “returns after taxes on distributions and sale of fund shares” may be higher than the figure shown under “returns before taxes” because the calculations assume the investor received a tax deduction for any loss incurred on the sale of shares.

Performance Table Closing [Text Block]

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Updated performance information is available through troweprice.com or may be obtained by calling 1-800-638-8790.

R Class Shares | T. Rowe Price Mid-Cap Growth Fund, Inc. | T. Rowe Price Mid-Cap Growth Fund-R Class

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

0.62%

Distribution and service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other expenses

rr_OtherExpensesOverAssets

0.16%

Total annual fund operating expenses

rr_ExpensesOverAssets

1.28%

1 year

rr_ExpenseExampleYear01

130

3 years

rr_ExpenseExampleYear03

406

5 years

rr_ExpenseExampleYear05

702

10 years

rr_ExpenseExampleYear10

1,545

2005

rr_AnnualReturn2005

14.29%

2006

rr_AnnualReturn2006

6.27%

2007

rr_AnnualReturn2007

17.10%

2008

rr_AnnualReturn2008

(39.97%)

2009

rr_AnnualReturn2009

44.79%

2010

rr_AnnualReturn2010

27.43%

2011

rr_AnnualReturn2011

(1.72%)

2012

rr_AnnualReturn2012

13.31%

2013

rr_AnnualReturn2013

36.22%

2014

rr_AnnualReturn2014

12.58%

Highest Quarterly Return, Label

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Best Quarter

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Sep. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

18.95%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(25.37%)

1 Year

rr_AverageAnnualReturnYear01

12.58%

5 Years

rr_AverageAnnualReturnYear05

16.83%

10 Years

rr_AverageAnnualReturnYear10

10.40%

R Class Shares | T. Rowe Price Mid-Cap Growth Fund, Inc. | Returns after taxes on distributions | T. Rowe Price Mid-Cap Growth Fund-R Class

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

10.43%

5 Years

rr_AverageAnnualReturnYear05

15.38%

10 Years

rr_AverageAnnualReturnYear10

9.24%

R Class Shares | T. Rowe Price Mid-Cap Growth Fund, Inc. | Returns after taxes on distributions and sale of fund shares | T. Rowe Price Mid-Cap Growth Fund-R Class

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

8.79%

5 Years

rr_AverageAnnualReturnYear05

13.58%

10 Years

rr_AverageAnnualReturnYear10

8.47%

R Class Shares | T. Rowe Price Mid-Cap Growth Fund, Inc. | Russell Midcap Growth Index (reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

11.90%

5 Years

rr_AverageAnnualReturnYear05

16.94%

10 Years

rr_AverageAnnualReturnYear10

9.43%

R Class Shares | T. Rowe Price Mid-Cap Growth Fund, Inc. | Lipper Mid-Cap Growth Funds Index

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

7.91%

5 Years

rr_AverageAnnualReturnYear05

14.72%

10 Years

rr_AverageAnnualReturnYear10

8.89%