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Related Party Transactions (Parenthetical) (Detail) - Kingsoft Tencent and Xiaomi Group	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Minimum [Member]
Related Party Transaction [Line Items]
Number of period for promotion services	1 year	1 year	1 year
Maximum [Member]
Related Party Transaction [Line Items]
Number of period for promotion services	2 years	2 years	2 years

X

- Definition

Number of period for promotion services.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=cmcm_KingsoftTencentAndXiaomiGroupMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: