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, 2019	Dec. 31, 2018
Disclosure Of Property Plant And Equipment [Line Items]
Future aggregate lease collection under operating lease for freehold plant, property and equipment	$ 471	$ 1,130
Year 1 [Member]
Disclosure Of Property Plant And Equipment [Line Items]
Future aggregate lease collection under operating lease for freehold plant, property and equipment	113	344
Year 2 [Member]
Disclosure Of Property Plant And Equipment [Line Items]
Future aggregate lease collection under operating lease for freehold plant, property and equipment	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	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	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	39
Onwards [Member]
Disclosure Of Property Plant And Equipment [Line Items]
Future aggregate lease collection under operating lease for freehold plant, property and equipment	96	$ 206
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	2,354
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	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	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	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	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	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	$ 1,224