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Income Tax - Analysis of Deferred Income Tax Assets and Liabilities in Consolidated Statements of Financial Position (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year $ 17,928.4 $ 16,806.4 $ 12,105.5
Recognized in Profit or loss 7,660.7 1,176.2 4,514.6
Recognized in Other Comprehensive Income 422.7 (21.0) 159.5
Effect of Exchange Rate Changes (53.6) (33.2) 26.8
Balance, End of Year 25,958.2 17,928.4 16,806.4
Deferred Income Tax Liabilities [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year (344.4) (233.3) (302.2)
Recognized in Profit or loss (1,385.5) (111.1) 32.7
Recognized in Other Comprehensive Income     36.2
Balance, End of Year (1,729.9) (344.4) (233.3)
Depreciation [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 13,547.2 11,839.2 8,401.3
Recognized in Profit or loss 5,824.0 1,727.8 3,430.4
Effect of Exchange Rate Changes (16.8) (19.8) 7.5
Balance, End of Year 19,354.4 13,547.2 11,839.2
Refund liability [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 2,150.4 2,594.0 1,637.7
Recognized in Profit or loss 1,606.1 (443.2) 955.0
Effect of Exchange Rate Changes (1.4) (0.4) 1.3
Balance, End of Year 3,755.1 2,150.4 2,594.0
Net defined benefit liability [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 1,016.3 1,084.9 975.3
Recognized in Profit or loss (96.3) (38.2) 6.3
Recognized in Other Comprehensive Income 422.1 (30.4) 103.3
Balance, End of Year 1,342.1 1,016.3 1,084.9
Unrealized loss on inventories [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 469.4 751.0 629.5
Recognized in Profit or loss 391.1 (280.8) 120.5
Effect of Exchange Rate Changes (2.0) (0.8) 1.0
Balance, End of Year 858.5 469.4 751.0
Deferred compensation cost [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 323.1 271.7 266.5
Recognized in Profit or loss 27.4 59.4 (4.7)
Effect of Exchange Rate Changes (20.2) (8.0) 9.9
Balance, End of Year 330.3 323.1 271.7
Investments in equity instruments at FVTOCI [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 65.7 56.2  
Recognized in Profit or loss   0.1  
Recognized in Other Comprehensive Income 0.6 9.4 56.2
Balance, End of Year 66.3 65.7 56.2
Investments in equity instruments at FVTOCI [member] | Deferred Income Tax Liabilities [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year     (95.4)
Recognized in Other Comprehensive Income     95.4
Others [member] | Deferred Income Tax Assets [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year 356.3 209.4 195.2
Recognized in Profit or loss (91.6) 151.1 7.1
Effect of Exchange Rate Changes (13.2) (4.2) 7.1
Balance, End of Year 251.5 356.3 209.4
Others [member] | Deferred Income Tax Liabilities [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year (10.8) (171.6) (37.3)
Recognized in Profit or loss (852.6) 160.8 (75.1)
Recognized in Other Comprehensive Income     (59.2)
Balance, End of Year (863.4) (10.8) (171.6)
Unrealized exchange gains [member] | Deferred Income Tax Liabilities [Member]      
Deferred income tax assets (liabilities)      
Balance, Beginning of Year (333.6) (61.7) (169.5)
Recognized in Profit or loss (532.9) (271.9) 107.8
Balance, End of Year $ (866.5) $ (333.6) $ (61.7)