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Investments Accounted for Using the Equity Method - Additional Information (Detail)
¥ in Thousands, shares in Thousands, $ in Thousands, $ in Thousands, $ in Thousands
 1 Months Ended 12 Months Ended
Jul. 31, 2019
TWD ($)
Jul. 31, 2019
USD ($)
Jul. 31, 2019
SGD ($)
May 31, 2019
TWD ($)
May 31, 2019
USD ($)
Apr. 30, 2019
TWD ($)
shares
Apr. 30, 2019
USD ($)
shares
Mar. 31, 2019
CNY (¥)
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
CNY (¥)
Mar. 31, 2019
TWD ($)
Mar. 31, 2019
USD ($)
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Gain recognized from remeasuring fair value of equity interest before business combination           $ 243,057 $ 8,126                  
Capital surplus                 $ 138,910,363   $ 143,276,664   $ 4,644,279      
Impairment losses on financial assets                 400,201 $ 13,380 521,010 $ 50,206        
Investments in joint ventures                 $ 279,702   $ 180,494   9,351      
ASE Test, Inc. [member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Percentage of ownership interest in associate           50.97% 50.97%                  
Percentage of ownership                 100.00% 100.00% 100.00%          
ASE Test, Inc. [member] | Private Placement [Member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Consideration paid in cash for shares           $ 250,000 $ 8,358                  
Shares subscribed | shares           100,000 100,000                  
Universal Global Technology Co., Limited [member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Consideration paid in cash for shares $ 1,824,608 $ 61,003 $ 79,863                          
Percentage of ownership 42.23% 42.23% 42.23%           75.30% 75.30% 74.60%          
ChipMos Technologies Inc [member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Percentage of ownership interest in associate                 20.48% 20.48% 20.47%          
Capital surplus                             $ 2,016 $ 67
Deca Technologies Inc. [member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Percentage of ownership interest in associate                 22.02% 22.02% 22.04%          
ASE Embedded Electronics Inc. [member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Gain recognized from remeasuring fair value of equity interest before business combination           $ 76,655 $ 2,563                  
Ordinary shares acquired       $ 150,000 $ 150,000                      
Consideration paid in cash for shares       $ 1,500,000 $ 50,150                      
Percentage of ownership interest in joint venture                     51.00%          
Percentage of ownership 100.00% 100.00% 100.00%                          
Investments in joint ventures                     $ 180,494          
Accumulated deficits compensated by capital reduction           $ 1,147,595 $ 38,368                  
ASE Embedded Electronics Inc. [member] | TDK Corporation [Member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Ordinary shares acquired   $ 6,000                            
SUMA-USI Electronics Co., Ltd. [member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Percentage of ownership interest in joint venture               49.00%                
Investments in joint ventures                 $ 283,236       $ 9,470 ¥ 65,170    
SUMA-USI Electronics Co., Ltd. [member] | Cancon Information Industry Co Ltd [Member]                                
Disclosure of Investments Accounted for Using Equity Method [line Items]                                
Consideration paid in cash for shares | ¥               ¥ 107,800