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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, 2018
TWD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Reconciliation of accounting profit multiplied by applicable tax rates [abstract]        
Income before tax $ 397,543.1   $ 396,161.9 $ 385,921.7
Income tax expense at the statutory rate 80,872.5   69,613.5 66,938.7
Tax effect of adjusting items:        
Nondeductible (deductible) items in determining taxable income 2,533.4   (1,415.9) (44.9)
Tax-exempt income (54,543.5)   (16,901.1) (19,595.0)
Additional income tax under the Alternative Minimum Tax Act 21,455.9      
Additional income tax on unappropriated earnings 16,294.5   28,183.5 30,046.8
Effect of tax rate changes on deferred income tax (1,474.8)   561.8  
The origination and reversal of temporary differences (3,072.5)   (4,336.1) (1,775.0)
Income tax credits (6,028.4)   (5,628.6) (4,940.2)
Remeasurement of operating loss carryforward       (0.4)
Tax effect of adjusting items 56,037.0   70,077.1 70,630.0
Income tax adjustments on prior years (21,753.0)   (19,107.0) (16,628.1)
Other income tax adjustments 152.9   152.8 122.5
Income tax expense recognized in profit or loss $ 34,436.9 $ 1,125.1 $ 51,122.9 $ 54,124.4