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Bonds Payable - Summary of Bonds Payable (Detail)
$ in Millions, $ in Millions
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Bonds Payable [Line Items]      
Less: Current portion $ (58,401.1)   $ (38,100.0)
Noncurrent bonds payable 91,800.0 $ 3,097.2 153,093.6
Cost [member]      
Bonds Payable [Line Items]      
Unsecured bonds 150,207.8   191,228.9
Cost [member] | Taiwan [member]      
Bonds Payable [Line Items]      
Unsecured bonds 116,100.0   154,200.0
Cost [member] | Overseas [member]      
Bonds Payable [Line Items]      
Unsecured bonds 34,107.8   37,028.9
Discounts on bonds payable [member]      
Bonds Payable [Line Items]      
Unsecured bonds $ 6.7   $ 35.3