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Income Tax - Reconciliation of Income before Income Tax and Income Tax Expense Recognized in Profit or Loss (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Reconciliation of accounting profit multiplied by applicable tax rates [abstract]        
Income before tax $ 389,862.1   $ 397,543.1 $ 396,161.9
Income tax expense at the statutory rate 79,056.5   80,872.5 69,613.5
Tax effect of adjusting items:        
Nondeductible (deductible) items in determining taxable income (4,183.5)   2,533.4 (1,415.9)
Tax-exempt income (39,808.1)   (54,543.5) (16,901.1)
Additional income tax under the Alternative Minimum Tax Act 10,367.9   21,455.9  
Additional income tax on unappropriated earnings 7,628.1   16,294.5 28,183.5
Effect of tax rate changes on deferred income tax     (1,474.8) 561.8
The origination and reversal of temporary differences (1,065.1)   (3,072.5) (4,336.1)
Income tax credits (5,925.4)   (6,028.4) (5,628.6)
Tax effect of adjusting items 46,070.4   56,037.0 70,077.1
Income tax adjustments on prior years (10,193.8)   (21,753.0) (19,107.0)
Other income tax adjustments (41.5)   152.9 152.8
Income tax expense recognized in profit or loss $ 35,835.1 $ 1,198.1 $ 34,436.9 $ 51,122.9