485BPOS

Securities Act of 1933 Registration No. 333-139428

Investment Company Act of 1940 Registration No. 811-21990

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 x

o Pre-Effective Amendment No.  ______

x Post-Effective Amendment No.  28

and

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 x

x Amendment No.  28

Fidelity Commonwealth Trust II

 (Exact Name of Registrant as Specified in Charter)

245 Summer Street, Boston, Massachusetts 02210

(Address of Principal Executive Offices)(Zip Code)

Registrant’s Telephone Number: 617-563-7000

Marc Bryant, Secretary

245 Summer Street

Boston, Massachusetts 02210

(Name and Address of Agent for Service)

It is proposed that this filing will become effective immediately upon filing pursuant to paragraph (b).

EXHIBIT INDEX

Exhibit	Exhibit No.
XBRL Instance Document	EX-101.INS
XBRL Taxonomy Extension Schema Document	EX-101.SCH
XBRL Taxonomy Extension Calculation Linkbase Document	EX-101.CAL
XBRL Taxonomy Extension Definition Linkbase Document	EX-101.DEF
XBRL Taxonomy Extension Labels Linkbase Document	EX-101.LAB
XBRL Taxonomy Extension Presentation Linkbase Document	EX-101.PRE

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for the effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment No. 28 to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, and Commonwealth of Massachusetts, on the 17th day of May 2018.

	Fidelity Commonwealth Trust II
	By	/s/Stacie M. Smith
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||		Stacie M. Smith, President

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

(Signature)		(Title)	(Date)
/s/Stacie M. Smith		President and Treasurer	May 17, 2018
Stacie M. Smith		(Principal Executive Officer)
/s/Howard J. Galligan III		Chief Financial Officer	May 17, 2018
Howard J. Galligan III		(Principal Financial Officer)
/s/James C. Curvey	*	Trustee	May 17, 2018
James C. Curvey
/s/Dennis J. Dirks	*	Trustee	May 17, 2018
Dennis J. Dirks
/s/Donald F. Donahue	*	Trustee	May 17, 2018
Donald F. Donahue
/s/Alan J. Lacy	*	Trustee	May 17, 2018
Alan J. Lacy
/s/Ned C. Lautenbach	*	Trustee	May 17, 2018
Ned C. Lautenbach
/s/Joseph Mauriello	*	Trustee	May 17, 2018
Joseph Mauriello
/s/Charles S. Morrison	*	Trustee	May 17, 2018
Charles S. Morrison
/s/Cornelia M. Small	*	Trustee	May 17, 2018
Cornelia M. Small
/s/Garnett A. Smith	*	Trustee	May 17, 2018
Garnett A. Smith
/s/David M. Thomas	*	Trustee	May 17, 2018
David M. Thomas
/s/Michael E. Wiley	*	Trustee	May 17, 2018
Michael E. Wiley

*	By:	/s/Megan C. Johnson
		Megan C. Johnson, pursuant to powers of attorney dated March 2, 2018 and filed herewith.

POWER OF ATTORNEY

We, the undersigned Directors or Trustees, as the case may be, of the following investment companies:

Fidelity Advisor Series I Fidelity Advisor Series VII Fidelity Advisor Series VIII Fidelity Beacon Street Trust Fidelity Capital Trust Fidelity Central Investment Portfolios LLC Fidelity Commonwealth Trust Fidelity Commonwealth Trust II Fidelity Concord Street Trust Fidelity Congress Street Fund Fidelity Contrafund Fidelity Covington Trust Fidelity Destiny Portfolios Fidelity Devonshire Trust Fidelity Exchange Fund Fidelity Financial Trust Fidelity Hanover Street Trust Fidelity Hastings Street Trust Fidelity Investment Trust Fidelity Magellan Fund Fidelity Mt. Vernon Street Trust Fidelity Puritan Trust Fidelity Securities Fund Fidelity Select Portfolios Fidelity Summer Street Trust Fidelity Trend Fund Variable Insurance Products Fund Variable Insurance Products Fund II Variable Insurance Products Fund III Variable Insurance Products Fund IV

in addition to any other investment company for which Fidelity Management & Research Company (“FMR”) or an affiliate acts as investment adviser and for which the undersigned individuals serve as Directors or Trustees (collectively, the “Funds”), hereby revoke all previous powers of attorney we have given to sign and otherwise act in our names and behalf in matters involving any investment company for which FMR or an affiliate acts as investment adviser and hereby constitute and appoint Thomas C. Bogle, John V. O’Hanlon, Robert W. Helm, Megan C. Johnson, and Anthony H. Zacharski, each of them singly, our true and lawful attorneys-in-fact, with full power of substitution, and with full power to each of them, to sign for us and in our names in the appropriate capacities, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in our names and behalf in connection therewith as said attorneys–in-fact deem necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  We hereby ratify and confirm all that said attorneys-in-fact or their substitutes may do or cause to be done by virtue hereof.  This power of attorney is effective for all documents filed on or after March 2, 2018.

WITNESS our hands on this second day of March 2018.

/s/ James C. Curvey	/s/ Joseph Mauriello
James C. Curvey	Joseph Mauriello
/s/ Dennis J. Dirks	/s/ Charles S. Morrison
Dennis J. Dirks	Charles S. Morrison
/s/ Donald F. Donahue	/s/ Cornelia M. Small
Donald F. Donahue	Cornelia M. Small
/s/ Alan J. Lacy	/s/ Garnett A. Smith
Alan J. Lacy	Garnett A. Smith
/s/ Ned C. Lautenbach	/s/ David M. Thomas
Ned C. Lautenbach	David M. Thomas

POWER OF ATTORNEY

I, the undersigned Director or Trustee, as the case may be, of the following investment companies:

Fidelity Advisor Series I Fidelity Advisor Series VII Fidelity Beacon Street Trust Fidelity Capital Trust Fidelity Central Investment Portfolios LLC Fidelity Commonwealth Trust Fidelity Commonwealth Trust II Fidelity Congress Street Fund Fidelity Contrafund Fidelity Covington Trust Fidelity Destiny Portfolios Fidelity Devonshire Trust Fidelity Exchange Fund Fidelity Financial Trust Fidelity Hanover Street Trust Fidelity Hastings Street Trust Fidelity Magellan Fund Fidelity Mt. Vernon Street Trust Fidelity Select Portfolios Fidelity Summer Street Trust Fidelity Trend Fund Variable Insurance Products Fund II Variable Insurance Products Fund III Variable Insurance Products Fund IV

in addition to any other investment company for which Fidelity Management & Research Company (“FMR”) or an affiliate acts as investment adviser and for which the undersigned individual serves as Director or Trustee (collectively, the “Funds”), hereby revokes all previous powers of attorney I have given to sign and otherwise act in my name and behalf in matters involving any investment company for which FMR or an affiliate acts as investment adviser and hereby constitute and appoint Thomas C. Bogle, John V. O’Hanlon, Robert W. Helm, Megan C. Johnson, and Anthony H. Zacharski, each of them singly, my true and lawful attorneys-in-fact, with full power of substitution, and with full power to each of them, to sign for me and in my name in the appropriate capacities, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in my name and behalf in connection therewith as said attorneys–in-fact deem necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  I hereby ratify and confirm all that said attorneys-in-fact or their substitutes may do or cause to be done by virtue hereof.  This power of attorney is effective for all documents filed on or after March 2, 2018.

WITNESS our hands on this second day of March 2018.

/s/ Michael E. Wiley    

Michael E. Wiley

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Feb. 28, 2018
Registrant Name	dei_EntityRegistrantName	Fidelity Commonwealth Trust II
Central Index Key	dei_EntityCentralIndexKey	0001364923
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Apr. 27, 2018
Document Effective Date	dei_DocumentEffectiveDate	Apr. 28, 2018
Prospectus Date	rr_ProspectusDate	Apr. 28, 2018

XML

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund

Fund Summary

Fund:
Fidelity® Small Cap Enhanced Index Fund

Investment Objective

The fund seeks capital appreciation.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Small Cap Enhanced Index Fund}	02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 Fidelity® Small Cap Enhanced Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

(expenses that you pay each year as a % of the value of your investment)

Annual Operating Expenses {- Fidelity® Small Cap Enhanced Index Fund}	02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 Fidelity® Small Cap Enhanced Index Fund Fidelity Small Cap Enhanced Index Fund-Default
Management fee	0.64%	[1]
Distribution and/or Service (12b-1) fees	none
Other expenses	none	[1]
Total annual operating expenses	0.64%	[1]

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Sell All Shares

Expense Example {- Fidelity® Small Cap Enhanced Index Fund}	02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 Fidelity® Small Cap Enhanced Index Fund Fidelity Small Cap Enhanced Index Fund-Default USD ($)
1 year	$ 65
3 years	205
5 years	357
10 years	$ 798

Portfolio Turnover

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 operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 100% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in common stocks included in the Russell 2000^® Index, which is a market capitalization-weighted index designed to measure the performance of the small-cap segment of the U.S. equity market.

• Generally using computer-aided, quantitative analysis of historical valuation, growth, profitability, and other factors to select a broadly diversified group of stocks that may have the potential to provide a higher total return than that of the Russell 2000^® Index.

• Investing in domestic and foreign issuers.

• Lending securities to earn income for the fund.

Principal Investment Risks

• Stock Market Volatility. Stock markets are volatile and can decline significantly in response to adverse issuer, political, regulatory, market, or economic developments. Different parts of the market, including different market sectors, and different types of securities can react differently to these developments.

• Foreign Exposure. Foreign markets can be more volatile than the U.S. market due to increased risks of adverse issuer, political, regulatory, market, or economic developments and can perform differently from the U.S. market.

• Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Quantitative Investing. Securities selected using quantitative analysis can perform differently from the market as a whole as a result of the factors used in the analysis, the weight placed on each factor, and changes in the factors' historical trends.

• Small Cap Investing. The value of securities of smaller, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. You could lose money by investing in the fund.

Performance

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time. The index description appears in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

Visit www.fidelity.com for more recent performance information.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	18.34%	June 30, 2009
Lowest Quarter Return	-24.46%	December 31, 2008
Year-to-Date Return	-0.14%	March 31, 2018

Average Annual Returns

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan). Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Small Cap Enhanced Index Fund} - 02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 - Fidelity® Small Cap Enhanced Index Fund	Past 1 year	Past 5 years	Past 10 years
Fidelity Small Cap Enhanced Index Fund-Default | Return Before Taxes	7.22%	13.59%	8.72%
Fidelity Small Cap Enhanced Index Fund-Default | After Taxes on Distributions	5.12%	11.95%	7.60%
Fidelity Small Cap Enhanced Index Fund-Default | After Taxes on Distributions and Sales	5.71%	10.51%	6.91%
Russell 2000® Index(reflects no deduction for fees, expenses, or taxes)	14.65%	14.12%	8.71%

[1] Adjusted to reflect current fees.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Commonwealth Trust II

Prospectus Date

rr_ProspectusDate

Apr. 28, 2018

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity® Small Cap Enhanced Index Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks capital appreciation.

Expense [Heading]

rr_ExpenseHeading

Fee Table

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder fees

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Operating Expenses

(expenses that you pay each year as a % 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 operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 100% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

100.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 80% of assets in common stocks included in the Russell 2000^® Index, which is a market capitalization-weighted index designed to measure the performance of the small-cap segment of the U.S. equity market.

• Generally using computer-aided, quantitative analysis of historical valuation, growth, profitability, and other factors to select a broadly diversified group of stocks that may have the potential to provide a higher total return than that of the Russell 2000^® Index.

• Investing in domestic and foreign issuers.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Stock Market Volatility. Stock markets are volatile and can decline significantly in response to adverse issuer, political, regulatory, market, or economic developments. Different parts of the market, including different market sectors, and different types of securities can react differently to these developments.

• Foreign Exposure. Foreign markets can be more volatile than the U.S. market due to increased risks of adverse issuer, political, regulatory, market, or economic developments and can perform differently from the U.S. market.

• Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Quantitative Investing. Securities selected using quantitative analysis can perform differently from the market as a whole as a result of the factors used in the analysis, the weight placed on each factor, and changes in the factors' historical trends.

• Small Cap Investing. The value of securities of smaller, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. You could lose money by investing in the fund.

Risk Lose Money [Text]

rr_RiskLoseMoney

You could lose money by investing in the fund.

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time. The index description appears in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

Visit www.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.fidelity.com

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

Past performance (before and after taxes) is not an indication of future performance.

Bar Chart [Heading]

rr_BarChartHeading

Year-by-Year Returns

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	18.34%	June 30, 2009
Lowest Quarter Return	-24.46%	December 31, 2008
Year-to-Date Return	-0.14%	March 31, 2018

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan).

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan). Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund | Fidelity Small Cap Enhanced Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.64%

^[1]

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.64%

^[1]

1 year

rr_ExpenseExampleYear01

$ 65

3 years

rr_ExpenseExampleYear03

205

5 years

rr_ExpenseExampleYear05

357

10 years

rr_ExpenseExampleYear10

$ 798

2008

rr_AnnualReturn2008

(33.67%)

2009

rr_AnnualReturn2009

23.60%

2010

rr_AnnualReturn2010

26.13%

2011

rr_AnnualReturn2011

(1.07%)

2012

rr_AnnualReturn2012

19.33%

2013

rr_AnnualReturn2013

38.66%

2014

rr_AnnualReturn2014

5.50%

2015

rr_AnnualReturn2015

(1.88%)

2016

rr_AnnualReturn2016

22.84%

2017

rr_AnnualReturn2017

7.22%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

(0.14%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

18.34%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(24.46%)

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund | Return Before Taxes | Fidelity Small Cap Enhanced Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Enhanced Index Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

7.22%

Past 5 years

rr_AverageAnnualReturnYear05

13.59%

Past 10 years

rr_AverageAnnualReturnYear10

8.72%

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund | After Taxes on Distributions | Fidelity Small Cap Enhanced Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Enhanced Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

5.12%

Past 5 years

rr_AverageAnnualReturnYear05

11.95%

Past 10 years

rr_AverageAnnualReturnYear10

7.60%

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund | After Taxes on Distributions and Sales | Fidelity Small Cap Enhanced Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Enhanced Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

5.71%

Past 5 years

rr_AverageAnnualReturnYear05

10.51%

Past 10 years

rr_AverageAnnualReturnYear10

6.91%

02.28 Fidelity Small Cap Enhanced Index Fund PRO-02 | Fidelity® Small Cap Enhanced Index Fund | Russell 2000® Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

14.65%

Past 5 years

rr_AverageAnnualReturnYear05

14.12%

Past 10 years

rr_AverageAnnualReturnYear10

8.71%

[1] Adjusted to reflect current fees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	Fidelity Commonwealth Trust II
Prospectus Date	rr_ProspectusDate	Apr. 28, 2018
Document Creation Date	dei_DocumentCreationDate	Apr. 27, 2018