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Bonds Payable - Major Terms of Domestic Unsecured Bonds (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Domestic Unsecured Bonds Hundred Two [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2012-01  
Issuance ending period 2019-01  
Coupon Rate 1.46% 1.46%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred Two [member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 7,000.0  
Domestic Unsecured Bonds Hundred And One One [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2012-08  
Issuance ending period 2019-08  
Coupon Rate 1.40% 1.40%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And One One [member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 9,000.0  
Domestic Unsecured Bonds Hundred And One Two [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2012-09  
Issuance ending period 2019-09  
Coupon Rate 1.39% 1.39%
Domestic unsecured bonds $ 9,000.0  
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And One Three [member]    
Bonds payable [line items]    
Issuance starting period 2012-10  
Issuance ending period 2022-10  
Coupon Rate 1.53% 1.53%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And One Three [member] | Taiwan [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 4,400.0  
Domestic Unsecured Bonds Hundred And One Four [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2013-01  
Issuance ending period 2020-01  
Coupon Rate 1.35% 1.35%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And One Four [member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2013-01  
Issuance ending period 2023-01  
Coupon Rate 1.49% 1.49%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And One Four [member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 10,000.0  
Domestic Unsecured Bonds Hundred And One Four [member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 3,000.0  
Domestic Unsecured Bonds Hundred And Two One [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2013-02  
Issuance ending period 2020-02  
Coupon Rate 1.38% 1.38%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Two One [member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2013-02  
Issuance ending period 2023-02  
Coupon Rate 1.50% 1.50%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Two One [member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 11,600.0  
Domestic Unsecured Bonds Hundred And Two One [member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 3,600.0  
Domestic Unsecured Bonds Hundred And Two Two [member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2013-07  
Issuance ending period 2020-07  
Coupon Rate 1.50% 1.50%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Two Two [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2013-07  
Issuance ending period 2023-07  
Coupon Rate 1.70% 1.70%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Two Two [member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 10,200.0  
Domestic Unsecured Bonds Hundred And Two Two [member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 3,500.0  
Domestic Unsecured Bonds Hundred And Two Three [member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2013-08  
Issuance ending period 2019-08  
Coupon Rate 1.52% 1.52%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Two Three [member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 8,500.0  
Domestic Unsecured Bonds Hundred And Two Four [member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2013-09  
Issuance ending period 2019-03  
Coupon Rate 1.60% 1.60%
Repayment and Interest Payment Bullet repayment; interest payable annually (interest for the six months prior to maturity will accrue on the basis of actual days and be repayable at maturity)  
Domestic Unsecured Bonds Hundred And Two Four [member] | Tranche D [member]    
Bonds payable [line items]    
Issuance starting period 2013-09  
Issuance ending period 2021-03  
Coupon Rate 1.85% 1.85%
Repayment and Interest Payment Bullet repayment; interest payable annually (interest for the six months prior to maturity will accrue on the basis of actual days and be repayable at maturity)  
Domestic Unsecured Bonds Hundred And Two Four [member] | Tranche E [member]    
Bonds payable [line items]    
Issuance starting period 2013-09  
Issuance ending period 2023-03  
Coupon Rate 2.05% 2.05%
Repayment and Interest Payment Bullet repayment; interest payable annually (interest for the six months prior to maturity will accrue on the basis of actual days and be repayable at maturity)  
Domestic Unsecured Bonds Hundred And Two Four [member] | Tranche F [member]    
Bonds payable [line items]    
Issuance starting period 2013-09  
Issuance ending period 2023-09  
Coupon Rate 2.10% 2.10%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Two Four [member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 1,400.0  
Domestic Unsecured Bonds Hundred And Two Four [member] | Taiwan [member] | Tranche D [member]    
Bonds payable [line items]    
Domestic unsecured bonds 2,600.0  
Domestic Unsecured Bonds Hundred And Two Four [member] | Taiwan [member] | Tranche E [member]    
Bonds payable [line items]    
Domestic unsecured bonds 5,400.0  
Domestic Unsecured Bonds Hundred And Two Four [member] | Taiwan [member] | Tranche F [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 2,600.0  
Domestic Unsecured Bonds Hundred And Nine One [Member]    
Bonds payable [line items]    
Issuance starting period 2020-09  
Issuance ending period 2060-09  
Coupon Rate 2.70% 2.70%
Repayment and Interest Payment Bullet repayment (callable on the 5th anniversary of the issue date and every anniversary thereafter); interest payable annually  
Domestic Unsecured Bonds Hundred And Nine One [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-03  
Issuance ending period 2025-03  
Coupon Rate 0.58% 0.58%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine One [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-03  
Issuance ending period 2027-03  
Coupon Rate 0.62% 0.62%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine One [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-03  
Issuance ending period 2030-03  
Coupon Rate 0.64% 0.64%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine One [Member] | Taiwan [member]    
Bonds payable [line items]    
Domestic unsecured bonds   $ 1,000.0
Domestic Unsecured Bonds Hundred And Nine One [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 3,000.0  
Domestic Unsecured Bonds Hundred And Nine One [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 10,500.0  
Domestic Unsecured Bonds Hundred And Nine One [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 10,500.0  
Domestic Unsecured Bonds Hundred And Nine Two [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-04  
Issuance ending period 2025-04  
Coupon Rate 0.52% 0.52%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Two [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-04  
Issuance ending period 2027-04  
Coupon Rate 0.58% 0.58%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Two [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-04  
Issuance ending period 2030-04  
Coupon Rate 0.60% 0.60%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Two [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 5,900.0  
Domestic Unsecured Bonds Hundred And Nine Two [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 10,400.0  
Domestic Unsecured Bonds Hundred And Nine Two [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 5,300.0  
Domestic Unsecured Bonds Hundred And Nine Three [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-05  
Issuance ending period 2025-05  
Coupon Rate 0.55% 0.55%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Three [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-05  
Issuance ending period 2027-05  
Coupon Rate 0.60% 0.60%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Three [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-05  
Issuance ending period 2030-05  
Coupon Rate 0.64% 0.64%
Repayment and Interest Payment Bullet repayment; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Three [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 4,500.0  
Domestic Unsecured Bonds Hundred And Nine Three [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 7,500.0  
Domestic Unsecured Bonds Hundred And Nine Three [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 2,400.0  
Domestic Unsecured Bonds Hundred And Nine Four [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-07  
Issuance ending period 2025-07  
Coupon Rate 0.58% 0.58%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Four [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-07  
Issuance ending period 2027-07  
Coupon Rate 0.65% 0.65%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Four [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-07  
Issuance ending period 2030-07  
Coupon Rate 0.67% 0.67%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Four [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 5,700.0  
Domestic Unsecured Bonds Hundred And Nine Four [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 6,300.0  
Domestic Unsecured Bonds Hundred And Nine Four [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 1,900.0  
Domestic Unsecured Bonds Hundred And Nine Five [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-09  
Issuance ending period 2025-09  
Coupon Rate 0.50% 0.50%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Five [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-09  
Issuance ending period 2027-09  
Coupon Rate 0.58% 0.58%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Five [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-09  
Issuance ending period 2030-09  
Coupon Rate 0.60% 0.60%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Five [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 4,800.0  
Domestic Unsecured Bonds Hundred And Nine Five [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 8,000.0  
Domestic Unsecured Bonds Hundred And Nine Five [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 2,800.0  
Domestic Unsecured Bonds Hundred And Nine Six [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-12  
Issuance ending period 2025-12  
Coupon Rate 0.40% 0.40%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Six [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-12  
Issuance ending period 2027-12  
Coupon Rate 0.44% 0.44%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Six [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-12  
Issuance ending period 2030-12  
Coupon Rate 0.48% 0.48%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Six [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 1,600.0  
Domestic Unsecured Bonds Hundred And Nine Six [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 5,600.0  
Domestic Unsecured Bonds Hundred And Nine Six [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 4,800.0  
Domestic Unsecured Bonds Hundred And Nine Seven [Member] | Tranche A [member]    
Bonds payable [line items]    
Issuance starting period 2020-12  
Issuance ending period 2025-12  
Coupon Rate 0.36% 0.36%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Seven [Member] | Tranche B [member]    
Bonds payable [line items]    
Issuance starting period 2020-12  
Issuance ending period 2027-12  
Coupon Rate 0.41% 0.41%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Seven [Member] | Tranche C [member]    
Bonds payable [line items]    
Issuance starting period 2020-12  
Issuance ending period 2030-12  
Coupon Rate 0.45% 0.45%
Repayment and Interest Payment Two equal installments in last two years; interest payable annually  
Domestic Unsecured Bonds Hundred And Nine Seven [Member] | Taiwan [member] | Tranche A [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 1,900.0  
Domestic Unsecured Bonds Hundred And Nine Seven [Member] | Taiwan [member] | Tranche B [member]    
Bonds payable [line items]    
Domestic unsecured bonds 10,200.0  
Domestic Unsecured Bonds Hundred And Nine Seven [Member] | Taiwan [member] | Tranche C [member]    
Bonds payable [line items]    
Domestic unsecured bonds $ 6,400.0