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Related Party Transactions (Parenthetical) (Detail) - Kingsoft Tencent and Xiaomi Group | 12 Months Ended
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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

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- 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: |