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Property, Plant and Equipment - Summary of Future Aggregate Lease Collection under Operating Lease for Freehold Plant, Property and Equipment - Assets Subject to Operating Leases (Detail) - TWD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment $ 434 $ 471
Year 1 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 115 113
Year 2 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 95 91
Year 3 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 75 71
Year 4 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 53 61
Year 5 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 38 39
Onwards [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 58 96
Assets Subject to Operating Leases [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 2,335 2,354
Assets Subject to Operating Leases [Member] | Year 1 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 347 302
Assets Subject to Operating Leases [Member] | Year 2 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 288 273
Assets Subject to Operating Leases [Member] | Year 3 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 231 234
Assets Subject to Operating Leases [Member] | Year 4 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 164 191
Assets Subject to Operating Leases [Member] | Year 5 [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment 125 130
Assets Subject to Operating Leases [Member] | Onwards [Member]    
Disclosure Of Property Plant And Equipment [Line Items]    
Future aggregate lease collection under operating lease for freehold plant, property and equipment $ 1,180 $ 1,224