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Cover Page
 12 Months Ended
Dec. 31, 2020
shares
Document Information [Line Items]  
Document Type 20-F
Amendment Flag false
Document Period End Date Dec. 31, 2020
Document Fiscal Year Focus 2020
Current Fiscal Year End Date --12-31
Document Fiscal Period Focus FY
Entity Registrant Name Silicon Motion Technology Corporation
Entity Filer Category Large Accelerated Filer
Entity File Number 000-51380
Entity Well-known Seasoned Issuer Yes
Document Annual Report true
Document Transition Report false
Document Shell Company Report false
Document Registration Statement false
Document Accounting Standard U.S. GAAP
Entity Shell Company false
Entity Voluntary Filers No
Entity Emerging Growth Company false
Entity Current Reporting Status Yes
Entity Interactive Data Current Yes
Entity Address, Address Line One Unit B, 16/F, Centre 600, 82 King Lam St, Cheung Sha Wan
Entity Address, Country HK
Entity Central Index Key 0001329394
Entity Address, City or Town Kowloon
Entity Incorporation, State or Country Code E9
Entity Common Stock, Shares Outstanding 138,167,852
Entity Address, Postal Zip Code 0000
ICFR Auditor Attestation Flag true
American Depositary Shares [Member]  
Document Information [Line Items]  
Title of 12(b) Security American Depositary Shares, each representing four ordinary shares
Security Exchange Name NASDAQ
Trading Symbol SIMO
Ordinary shares[Member]  
Document Information [Line Items]  
Title of 12(b) Security Ordinary shares, par value US$0.01 per share
No Trading Symbol Flag true
Business Contact [Member]  
Document Information [Line Items]  
Entity Address, Address Line One 690 N. McCarthy Blvd. Suite 200
Entity Address, Country US
Entity Address, City or Town Milpitas
Entity Address, State or Province CA
Contact Personnel Fax Number 1 408 519 7101
Country Region 1
City Area Code 408
Local Phone Number 519 7200
Entity Address, Postal Zip Code 95035
Contact Personnel Name Riyadh Lai