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Trade Notes and Accounts Receivable, Net - Provision Matrix Arising from Telecommunications and Project Business (Detail)
$ in Millions, $ in Millions
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Jan. 01, 2019
TWD ($)
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 22,622 $ 806 $ 26,408 $ 30,076
Less: Loss allowance (2,154)   (2,360)  
Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 24,776   28,768  
Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance (797)   (799)  
Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 16,806   20,080  
Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 16,009   19,281  
Project Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 50.00% 50.00%    
Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (1,320)   (1,502)  
Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 4,866   5,699  
Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 3,546   $ 4,197  
Not Past Due [Member] | Telecommunications Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 0.00% 0.00% 0.00%  
Not Past Due [Member] | Telecommunications Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 2.00% 2.00% 2.00%  
Not Past Due [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (56)   $ (56)  
Not Past Due [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 15,839   19,020  
Not Past Due [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 15,783   $ 18,964  
Not Past Due [Member] | Project Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 0.00% 0.00% 0.00%  
Not Past Due [Member] | Project Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 5.00% 5.00% 5.00%  
Not Past Due [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (20)   $ (3)  
Not Past Due [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 3,473   4,054  
Not Past Due [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 3,453   $ 4,051  
Less Than 30 Days [Member] | Telecommunications Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 2.00% 2.00% 0.00%  
Less Than 30 Days [Member] | Telecommunications Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 24.00% 24.00% 25.00%  
Less Than 30 Days [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (21)   $ (26)  
Less Than 30 Days [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 204   268  
Less Than 30 Days [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 183   $ 242  
Less Than 30 Days [Member] | Project Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 5.00% 5.00% 5.00%  
Less Than 30 Days [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (3)   $ (5)  
Less Than 30 Days [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 64   78  
Less Than 30 Days [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 61   $ 73  
31-60 Days [Member] | Telecommunications Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 3.00% 3.00% 0.00%  
31-60 Days [Member] | Telecommunications Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 68.00% 68.00% 68.00%  
31-60 Days [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (24)   $ (28)  
31-60 Days [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 51   75  
31-60 Days [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 27   $ 47  
31-60 Days [Member] | Project Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 10.00% 10.00% 10.00%  
31-60 Days [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (3)   $ (5)  
31-60 Days [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 27   52  
31-60 Days [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 24   $ 47  
61-90 Days [Member] | Telecommunications Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 11.00% 11.00% 0.00%  
61-90 Days [Member] | Telecommunications Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 83.00% 83.00% 83.00%  
61-90 Days [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (25)   $ (35)  
61-90 Days [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 31   47  
61-90 Days [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 6   $ 12  
61-90 Days [Member] | Project Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 30.00% 30.00% 30.00%  
61-90 Days [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (3)   $ (11)  
61-90 Days [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 9   30  
61-90 Days [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 6   $ 19  
91-120 Days [Member] | Telecommunications Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 28.00% 28.00% 11.00%  
91-120 Days [Member] | Telecommunications Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 90.00% 90.00% 90.00%  
91-120 Days [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (24)   $ (26)  
91-120 Days [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 30   41  
91-120 Days [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 6   $ 15  
91-120 Days [Member] | Project Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 50.00% 50.00% 50.00%  
91-120 Days [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (1)   $ (5)  
91-120 Days [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 2   12  
91-120 Days [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 1   $ 7  
121-180 Days [Member] | Telecommunications Business [Member] | Bottom of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 52.00% 52.00% 17.00%  
121-180 Days [Member] | Telecommunications Business [Member] | Top of Range [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 96.00% 96.00% 96.00%  
121-180 Days [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (21)   $ (27)  
121-180 Days [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 25   28  
121-180 Days [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 4   $ 1  
121-180 Days [Member] | Project Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 80.00% 80.00% 80.00%  
121-180 Days [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (2)   $ (1)  
121-180 Days [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 3   1  
121-180 Days [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 1   $ 0  
Over 180 Days [Member] | Telecommunications Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 100.00% 100.00% 100.00%  
Over 180 Days [Member] | Telecommunications Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (626)   $ (601)  
Over 180 Days [Member] | Telecommunications Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 626   601  
Over 180 Days [Member] | Telecommunications Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 0   $ 0  
Over 180 Days [Member] | Project Business [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Expected credit loss rate 100.00% 100.00% 100.00%  
Over 180 Days [Member] | Project Business [Member] | Loss allowance [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Less: Loss allowance $ (1,288)   $ (1,472)  
Over 180 Days [Member] | Project Business [Member] | Gross carrying amount [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable 1,288   1,472  
Over 180 Days [Member] | Project Business [Member] | Amortized Cost [Member]        
Disclosure of financial assets that are either past due or impaired [Line Items]        
Trade notes and accounts receivable $ 0   $ 0