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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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The increase (decrease) in the fair value measurement of assets. [Refer: At fair value [member]]

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The amount of risk exposure associated with financial instruments with a shared characteristic that identifies a concentration of risks. [Refer: Financial instruments, class [member]]

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Discount for lack of marketability percentage.

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Increase decrease expected credit loss

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Percentage of accounts receivable from the top ten customers.

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Risk exposure associated with instruments sharing characteristic, percentage change in risk

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Financial Instruments - Additional Information (Detail) - TWD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of detailed information about financial instruments [line items]
Percentage of accounts receivable from ten largest customers	79.00%	70.00%
Expected credit loss decreases	$ (1.1)
Level 3 [member]
Disclosure of detailed information about financial instruments [line items]
Discount for lack of marketability percentage	10.00%
Decrease in fair value due to lack of marketability increase by 1%	$ (31.4)
Percentage increase in the discount for lack of marketability	1.00%
Level 3 [member] \| Discount rate [member]
Disclosure of detailed information about financial instruments [line items]
Description of discount rates applied to cash flow projections	10.00%
Level 3 [member] \| Discount for lack of marketability [member]
Disclosure of detailed information about financial instruments [line items]
Description of discount rates applied to cash flow projections	10.00%
Level 3 [member] \| Discount for lack of control [member]
Disclosure of detailed information about financial instruments [line items]
Description of discount rates applied to cash flow projections	10.00%
Equity Price Risk [member]
Disclosure of detailed information about financial instruments [line items]
Assumed change in risk	(10.00%)
Assumed change in risk, monetary effect	$ (427.1)	$ (703.0)	$ (685.1)
Currency Risk [member]
Disclosure of detailed information about financial instruments [line items]
Assumed change in risk	(10.00%)
Assumed change in risk, monetary effect	$ (506.4)	(867.9)	(111.3)
Currency Risk [member] \| Other comprehensive income [member]
Disclosure of detailed information about financial instruments [line items]
Assumed change in risk, monetary effect	$ (315.6)	(265.9)
Interest rate risk [member] \| Available-for-sale Fixed Income Securities [member]
Disclosure of detailed information about financial instruments [line items]
Assumed change in risk	1.00%
Assumed change in risk, monetary effect	$ (2,450.0)	$ (2,119.7)	$ (1,600.9)
Decrease in net income	$ (247.8)