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Related Party Transactions - Additional Information (Detail) ¥ in Thousands, $ in Thousands | Dec. 31, 2017 CNY (¥) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 CNY (¥) | Dec. 31, 2015 CNY (¥)
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Allowance for doubtful accounts | ¥ 7,312 | $ 1,124 | ¥ 8,282 |

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- Definition Represents information pertaining to valuation allowance for trade and other receivables due to an Entity, from a related party, that are expected to be uncollectible. + References No definition available. + Details
Name: | cmcm_RelatedPartyTransactionAllowanceForDoubtfulAccountsReceivable
Namespace Prefix: | cmcm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant