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Organization and Principal Activities - Additional Information (Detail) ¥ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 CNY (¥)	Dec. 31, 2017 USD ($)	Oct. 02, 2017
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] | Exclusive technology development, support and consulting agreements
Variable Interest Entity [Line Items]
Minimum service fees as percentage of VIE pre-tax revenue	20.00%	20.00%
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] | Loan agreements
Variable Interest Entity [Line Items]
Loans granted to nominee shareholders by primary beneficiaries	¥ 16,800	$ 2,582
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] | Shareholder voting proxy agreements
Variable Interest Entity [Line Items]
Shareholder voting proxy agreement effective term	10 years	10 years
Period of notice to terminate the agreement	30 days	30 days
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] | Business operation agreements
Variable Interest Entity [Line Items]
Term of business operation agreement	10 years	10 years
Chief Executive Officer and Director
Variable Interest Entity [Line Items]
Percentage of voting right delegated			38.00%

X

- Definition

Represents the amount of loans granted by primary beneficiaries to the nominee shareholders pursuant to the loan agreements.

+ References

No definition available.

+ Details

Name:	cmcm_LoansGrantedToNomineeShareholdersByPrimaryBeneficiaries
Namespace Prefix:	cmcm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Represents the minimum fees to be paid to the beneficiaries as a percentage of the VIE pre-tax revenue.

+ References

No definition available.

+ Details

Name:	cmcm_MinimumServiceFeesAsPercentageOfViePreTaxRevenue
Namespace Prefix:	cmcm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of voting equity interests delegated.

+ References

No definition available.

+ Details

Name:	cmcm_PercentageOfVotingInterestsDelegated
Namespace Prefix:	cmcm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the notice period for termination of agreement to be provided with in expiration.

+ References

No definition available.

+ Details

Name:	cmcm_PeriodOfNoticeForTerminationBeforeExpiration
Namespace Prefix:	cmcm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The effective period of shareholder voting proxy agreements.

+ References

No definition available.

+ Details

Name:	cmcm_ShareholderVotingProxyAgreementsEffectivePeriod
Namespace Prefix:	cmcm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the term of business operation agreement.

+ References

No definition available.

+ Details

Name:	cmcm_TermOfBusinessOperationAgreement
Namespace Prefix:	cmcm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_TypeOfArrangementAxis=cmcm_ExclusiveTechnologyDevelopmentSupportAndConsultingAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_TypeOfArrangementAxis=cmcm_LoanAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_TypeOfArrangementAxis=cmcm_ShareholderVotingProxyAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_TypeOfArrangementAxis=cmcm_BusinessOperationAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_TitleOfIndividualAxis=cmcm_ChiefExecutiveOfficerAndDirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: