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Retirement Benefit Plans - Movements in Defined Benefit Obligation and Fair Value of Plan Assets (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Disclosure of defined benefit plans [Line Items]        
Net defined benefit liability (asset) at beginning of period $ 618   $ 7,088 $ 6,462
Current service cost 2,918   2,866 2,884
Interest expense/interest income (13)   27 102
Amounts recognized in profit or loss 2,905   2,893 2,986
Remeasurement on the net defined benefit liability        
Return on plan assets (excluding amounts included in net interest) 193   352 (136)
Actuarial (gain) losses recognized from changes in demographic assumptions 15   (124) 11
Actuarial (gains) losses recognized from changes in financial assumptions     1,715 (1)
Actuarial losses recognized from experience adjustments 1,816   100 357
Amounts recognized in other comprehensive income 2,024 $ 68 2,043 231
Contributions from employer (2,635)   (11,235) (2,435)
Benefits paid 0   0 0
Benefits paid directly by the Company (221)   (171) (156)
Net defined benefit liability (asset) at end of period 2,691   618 7,088
Present value of funded defined benefit obligation [Member]        
Disclosure of defined benefit plans [Line Items]        
Net defined benefit liability (asset) at beginning of period 34,572   30,882 27,958
Current service cost 2,918   2,866 2,884
Interest expense/interest income 506   600 546
Amounts recognized in profit or loss 3,424   3,466 3,430
Remeasurement on the net defined benefit liability        
Return on plan assets (excluding amounts included in net interest) 0   0 0
Actuarial (gain) losses recognized from changes in demographic assumptions 15   (124) 11
Actuarial (gains) losses recognized from changes in financial assumptions     1,715 (1)
Actuarial losses recognized from experience adjustments 1,816   100 357
Amounts recognized in other comprehensive income 1,831   1,691 367
Contributions from employer 0   0 0
Benefits paid (1,943)   (1,296) (717)
Benefits paid directly by the Company (221)   (171) (156)
Net defined benefit liability (asset) at end of period 37,663   34,572 30,882
Fair value of plan assets [Member]        
Disclosure of defined benefit plans [Line Items]        
Net defined benefit liability (asset) at beginning of period 33,954   23,794 21,496
Current service cost 0   0 0
Interest expense/interest income 519   573 444
Amounts recognized in profit or loss 519   573 444
Remeasurement on the net defined benefit liability        
Return on plan assets (excluding amounts included in net interest) (193)   (352) 136
Actuarial (gain) losses recognized from changes in demographic assumptions 0   0 0
Actuarial (gains) losses recognized from changes in financial assumptions     0 0
Actuarial losses recognized from experience adjustments 0   0 0
Amounts recognized in other comprehensive income (193)   (352) 136
Contributions from employer 2,635   11,235 2,435
Benefits paid (1,943)   (1,296) (717)
Benefits paid directly by the Company 0   0 0
Net defined benefit liability (asset) at end of period $ 34,972   $ 33,954 $ 23,794