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Income Tax - Additional Information (Detail) $ in Thousands, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2017 TWD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 TWD ($)	Dec. 31, 2015	Dec. 31, 2017 USD ($)
Disclosure of income taxes [line items]
Applied tax rate	20.00%	17.00%	17.00%	17.00%	17.00%
Income tax on unappropriated earnings	5.00%	10.00%	10.00%
Increase in deferred tax assets		$ 201,965	$ 6,814
Increase in deferred tax liabilities		788,556				$ 26,604
Taxable temporary differences associated with the investments in subsidiaries for which no deferred tax liabilities recognized		16,401,422		$ 14,417,873		553,354
Balance of Imputation Credit Account		$ 4,003,283		$ 3,328,374		$ 135,064
Ratio of distribution of earnings				10.01%
Subsidiaries [member] | China [member]
Disclosure of income taxes [line items]
Applied tax rate		25.00%	25.00%	25.00%	25.00%
Reduction tax rate		15.00%	15.00%

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Line items represent concepts included in income taxes.

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Income tax rate on unappropriated earnings

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Increase decrease in deferred tax liability.

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Percentage of dividends recognised as distributions to owners.

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- Definition

The applicable income tax rate.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The amount of regulatory deferral account credit balances. [Refer: Regulatory deferral account balances [member]]

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The aggregate amount of temporary differences associated with investments in subsidiaries, branches and associates and interests in joint arrangements, for which deferred tax liabilities have not been recognised as a result of satisfying both of the following conditions: (a) the parent, investor, joint venturer or joint operator is able to control the timing of the reversal of the temporary difference; and (b) it is probable that the temporary difference will not reverse in the foreseeable future. [Refer: Associates [member]; Subsidiaries [member]; Temporary differences [member]; Investments in subsidiaries]

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