Investments Accounted for Using Equity Method - Market Prices of Investments Accounted for Using Equity Method in Publicly Traded Stocks Calculated by Closing Price at End of Reporting Period (Detail) - Level 1 [member] - TWD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Vanguard International Semiconductor Corporation [member]
Disclosure of associates [line items]
Closing Price in Active Markets	$ 27,621.3	$ 30,638.8
Global Unichip Corporation [member]
Disclosure of associates [line items]
Closing Price in Active Markets	9,617.7	11,905.4
Xintec Inc. [member]
Disclosure of associates [line items]
Closing Price in Active Markets	$ 3,783.6	$ 9,180.8

Investments Accounted for Using Equity Method - Market Prices of Investments Accounted for Using Equity Method in Publicly Traded Stocks Calculated by Closing Price at End of Reporting Period (Detail) - Level 1 [member] - TWD ($)
$ in Millions

Dec. 31, 2018

Dec. 31, 2017

Vanguard International Semiconductor Corporation [member]

Disclosure of associates [line items]

Closing Price in Active Markets

$ 27,621.3

$ 30,638.8

Global Unichip Corporation [member]

Disclosure of associates [line items]

Closing Price in Active Markets

9,617.7

11,905.4

Xintec Inc. [member]

Disclosure of associates [line items]

Closing Price in Active Markets

$ 3,783.6

$ 9,180.8