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Application of New and Revised International Financial Reporting Standards as Issued by the International Accounting Standards Board ("IASB") ( Collectively, "IFRSs") - Summary of Assets and Liablities (Detail) $ in Thousands, $ in Thousands | Jan. 01, 2018 USD ($) | Jan. 01, 2018 TWD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 TWD ($) | Jan. 01, 2018 TWD ($) | Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($)
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Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Investments accounted for using the equity method |  |  | $ 304,224 | $ 9,312,308 |  | $ 48,753,751 | $ 49,824,690
FVTPL |  |  | $ 258,035 | $ 7,898,458 |  | $ 5,223,067 |
IAS 39 [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Investments accounted for using the equity method | $ 1,592,739 |  |  |  | $ 48,753,751 |  |
Financial assets | 1,763,372 |  |  |  | 53,976,818 |  |
IAS 39 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
FVTPL | 170,633 |  |  |  | 5,223,067 |  |
Total fair value through profit or loss | 170,633 |  |  |  | 5,223,067 |  |
Reclassifications [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 72,270 |  |  |  | 2,212,165 |  |
Reclassifications [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Add: Reclassification from available-for-sale (IAS 39) - required reclassification | 9,919 | $ 303,616 |  |  |  |  |
Total fair value through profit or loss | 9,919 |  |  |  | 303,616 |  |
Reclassifications [member] | Financial assets at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Debt instruments |  |  |  |  |  |  |
Add: Reclassification from other financial assets | 32,669 | 1,000,000 |  |  |  |  |
Equity instruments |  |  |  |  |  |  |
Add: Reclassification from available-for-sale (IAS 39) | 29,682 | 908,549 |  |  |  |  |
Total equity and debt instruments measured at fair value through other comprehensive income | 62,351 | 1,908,549 |  |  |  |  |
Remeasurements [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Investments accounted for using the equity method | (85) |  |  |  | (2,586) |  |
Financial assets | 2,529 |  |  |  | 77,414 |  |
Remeasurements [member] | Financial assets at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Debt instruments |  |  |  |  |  |  |
Add: Reclassification from other financial assets | 2,614 | 80,000 |  |  |  |  |
Equity instruments |  |  |  |  |  |  |
Total equity and debt instruments measured at fair value through other comprehensive income | 2,614 | 80,000 |  |  |  |  |
IFRS9 [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Investments accounted for using the equity method | 1,592,655 |  |  |  | 48,751,165 |  |
Financial assets | 1,838,170 |  |  |  | 56,266,397 |  |
IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Total fair value through profit or loss | 180,551 |  |  |  | 5,526,683 |  |
IFRS9 [member] | Financial assets at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Equity instruments |  |  |  |  |  |  |
Total equity and debt instruments measured at fair value through other comprehensive income | 64,964 | 1,988,549 |  |  |  |  |
Cash and cash equivalents [member] | IAS 39 [member] | Loans and receivables [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 1,505,327 |  |  |  | 46,078,066 |  |
Cash and cash equivalents [member] | IFRS9 [member] | Amortized cost [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 1,505,327 |  |  |  | 46,078,066 |  |
Derivatives [member] | IAS 39 [member] | Held for trading [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 3,981 |  |  |  | 121,863 |  |
Derivatives [member] | IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 3,981 |  |  |  | 121,863 |  |
Equity instruments1 [member] | IAS 39 [member] | Held for trading [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 144,094 |  |  |  | 4,410,732 |  |
Equity instruments1 [member] | IAS 39 [member] | Available-for-sale [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 9,141 |  |  |  | 279,791 |  |
Equity instruments1 [member] | IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 144,094 |  |  |  | 4,410,732 |  |
Equity instruments2 [member] | IAS 39 [member] | Available-for-sale [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 29,681 |  |  |  | 908,549 |  |
Equity instruments2 [member] | IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 9,141 |  |  |  | 279,791 |  |
Equity instruments2 [member] | IFRS9 [member] | Financial assets measured at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 29,681 |  |  |  | 908,549 |  |
Open-end mutual funds [member] | IAS 39 [member] | Held for trading [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 19,274 |  |  |  | 589,976 |  |
Open-end mutual funds [member] | IAS 39 [member] | Available-for-sale [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 778 |  |  |  | 23,825 |  |
Open-end mutual funds [member] | IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 19,274 |  |  |  | 589,976 |  |
Debt instruments [member] | IAS 39 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 3,283 |  |  |  | 100,496 |  |
Debt instruments [member] | IAS 39 [member] | Other financial assets [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 32,669 |  |  |  | 1,000,000 |  |
Debt instruments [member] | IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 3,283 |  |  |  | 100,496 |  |
Debt instruments [member] | IFRS9 [member] | Financial assets measured at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 35,283 |  |  |  | 1,080,000 |  |
Time deposits with original maturity of over three months, pledged time deposits and guarantee deposit [member] | IAS 39 [member] | Loans and receivables [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 13,248 |  |  |  | 405,520 |  |
Time deposits with original maturity of over three months, pledged time deposits and guarantee deposit [member] | IFRS9 [member] | Amortized cost [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 13,248 |  |  |  | 405,520 |  |
Trade receivables and other receivables [member] | IAS 39 [member] | Loans and receivables [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 1,837,722 |  |  |  | 56,252,661 |  |
Trade receivables and other receivables [member] | IFRS9 [member] | Amortized cost [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 1,837,722 |  |  |  | 56,252,661 |  |
Open-end mutual funds [member] | IFRS9 [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Financial assets | 778 |  |  |  | 23,825 |  |
Retained earnings [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Investments accounted for using the equity method | (5,344) |  |  |  | (163,579) |  |
Financial assets | 11,907 |  |  |  | 364,467 |  |
Retained earnings [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Add: Reclassification from available-for-sale (IAS 39) - required reclassification | 3,615 | 110,648 |  |  |  |  |
Total fair value through profit or loss | 3,615 |  |  |  | 110,648 |  |
Retained earnings [member] | Financial assets at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Equity instruments |  |  |  |  |  |  |
Add: Reclassification from available-for-sale (IAS 39) | 13,636 | 417,398 |  |  |  |  |
Total equity and debt instruments measured at fair value through other comprehensive income | 13,636 | 417,398 |  |  |  |  |
Other equity securities [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Investments accounted for using the equity method | 5,259 |  |  |  | 160,993 |  |
Financial assets | (9,378) |  |  |  | (287,053) |  |
Other equity securities [member] | Mandatorily at fair value through profit or loss [member] |  |  |  |  |  |  |
Disclosure of financial assets at date of initial application of IFRS 9 [line items] |  |  |  |  |  |  |
Add: Reclassification from available-for-sale (IAS 39) - required reclassification | (3,615) | (110,648) |  |  |  |  |
Total fair value through profit or loss | (3,615) |  |  |  | $ (110,648) |  |
Other equity securities [member] | Financial assets at fair value through other comprehensive income, category [member] |  |  |  |  |  |  |
Debt instruments |  |  |  |  |  |  |
Add: Reclassification from other financial assets | 2,614 | 80,000 |  |  |  |  |
Equity instruments |  |  |  |  |  |  |
Add: Reclassification from available-for-sale (IAS 39) | (13,636) | (417,398) |  |  |  |  |
Total equity and debt instruments measured at fair value through other comprehensive income | $ (11,022) | $ (337,398) |  |  |  |  |

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