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Significant Contingent Liabilities and Unrecognized Commitments - Additional Information (Detail) - TWD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Aug. 15, 1996
Commitments And Contingencies [Line Items]      
Unused letters of credit   $ 50  
Commitment amount   1,614  
Contractual Consideration Payment [Member]      
Commitments And Contingencies [Line Items]      
Contractual consideration payment $ 567    
Piping Fund [Member]      
Commitments And Contingencies [Line Items]      
Commitment amount     $ 2,000
Commitment to contribute to a Piping Fund, amount contributed     $ 1,000
Commitment to contribute to a Piping Fund, remaining amount   1,000  
Land and buildings [Member]      
Commitments And Contingencies [Line Items]      
Commitments for acquisitions of property, plant and equipment   174  
Telecommunications equipment [Member]      
Commitments And Contingencies [Line Items]      
Commitments for acquisitions of property, plant and equipment   $ 15,760