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Summary of significant accounting policies (Tables)
 12 Months Ended
Dec. 31, 2020
Text block [abstract]  
Summary of Ownership Interests in Subsidiaries
          
Percentage of

Ownership (%)
     
          
December 31,
     
Name of investor
  
Name of investee
 
Main business
  
Location
 
2019
   
2020
   
Note
 
The Company
  ChipMOS U.S.A., Inc.
(“ChipMOS USA”)		 Marketing of semiconductors and electronic related products  San Jose, USA  100    100   
The Company
  ChipMOS
TECHNOLOGIES
(BVI) LTD.
(“ChipMOS BVI”)
 Holding company  British Virgin
Islands		  100    100   
ChipMOS BVI
  ChipMOS
SEMICONDUCTORS
(Shanghai) LTD.
(“ChipMOS
Shanghai”)		 Marketing of semiconductors and electronic related products  Shanghai,
People’s
Republic of
China
(“PRC”)		  —      100    Note 
Estimated Useful Life for Depreciation of Individual Assets
Buildings
  
5 to 51 years
Machinery and equipment
  
2 to 8 years
Tools
  
2 to 4 years
Others
  
2 to 6 years