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Income Tax - Movements of Deferred Income Tax Assets and Liabilities (Detail) - TWD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income tax [Line Items] |  |  |
Effect of Retrospective Applicationof IFRS 9 (Note 5) |  | $ (883) |
Recognized in Profit or Loss | $ (208) | 101 | $ (1,404)
Recognized in Other Comprehensive Income | 450 | 347 | 349
Deferred tax asset [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 2,730 | 2,322 | 2,061
Recognized in Profit or Loss | 374 | 64 | (86)
Recognized in Other Comprehensive Income | 450 | 344 | 347
Ending balance | 3,554 | 2,730 | 2,322
Deferred tax asset [Member] | Present value of funded defined benefit obligation [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 1,723 | 1,374 | 1,206
Recognized in Profit or Loss | 134 | 5 | (179)
Recognized in Other Comprehensive Income | 450 | 344 | 347
Ending balance | 2,307 | 1,723 | 1,374
Deferred tax asset [Member] | Allowance for doubtful accounts [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 289 | 230 | 169
Recognized in Profit or Loss | 146 | 59 | 61
Ending balance | 435 | 289 | 230
Deferred tax asset [Member] | Share of profits of associates and joint ventures accounted for using equity method [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 331 | 330 | 325
Recognized in Profit or Loss | 58 | 1 | 5
Ending balance | 389 | 331 | 330
Deferred tax asset [Member] | Impairment loss on property, plant and equipment [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 112 | 122 | 44
Recognized in Profit or Loss | (19) | (10) | 78
Ending balance | 93 | 112 | 122
Deferred tax asset [Member] | Deferred revenue [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 106 | 117 | 136
Recognized in Profit or Loss | 5 | (11) | (19)
Ending balance | 111 | 106 | 117
Deferred tax asset [Member] | Valuation loss on inventory [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 23 | 20 | 33
Recognized in Profit or Loss | 65 | 3 | (13)
Ending balance | 88 | 23 | 20
Deferred tax asset [Member] | Accrued award credits liabilities [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 15 | 20 | 22
Recognized in Profit or Loss | (1) | (5) | (2)
Ending balance | 14 | 15 | 20
Deferred tax asset [Member] | Warranties [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 22 | 19 | 18
Recognized in Profit or Loss | 4 | 3 | 1
Ending balance | 26 | 22 | 19
Deferred tax asset [Member] | Property, plant and equipment [member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 2 | 2 | 2
Ending balance | 2 | 2 | 2
Deferred tax asset [Member] | Unrealised foreign exchange gains (losses) [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 17 | 0 | 18
Effect of Retrospective Applicationof IFRS 9 (Note 5) | 0 |  |
Recognized in Profit or Loss | (16) | 17 | (18)
Ending balance | 1 | 17 | 0
Deferred tax asset [Member] | Others [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 29 | 34 | 40
Recognized in Profit or Loss | 8 | (5) | (6)
Ending balance | 37 | 29 | 34
Deferred tax asset [Member] | Temporary differences [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 2,684 | 2,268 | 2,013
Recognized in Profit or Loss | 379 | 72 | (92)
Recognized in Other Comprehensive Income | 450 | 344 | 347
Ending balance | 3,513 | 2,684 | 2,268
Deferred tax asset [Member] | Unused tax losses [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 46 | 54 | 48
Recognized in Profit or Loss | (5) | (8) | 6
Ending balance | 41 | 46 | 54
Deferred tax asset [Member] | Trade-in right [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 15 | 0 |
Effect of Retrospective Applicationof IFRS 9 (Note 5) | 0 |  |
Recognized in Profit or Loss | (5) | 15 |
Ending balance | 10 | 15 | 0
Deferred tax liability [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (1,430) | (1,464) | (148)
Effect of Retrospective Applicationof IFRS 9 (Note 5) | 1 |  |
Recognized in Profit or Loss | (563) | 31 | (1,318)
Recognized in Other Comprehensive Income |  | 3 | 2
Ending balance | (1,992) | (1,430) | (1,464)
Deferred tax liability [Member] | Present value of funded defined benefit obligation [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (1,265) | (1,269) | (1)
Recognized in Profit or Loss | (567) | 4 | (1,268)
Ending balance | (1,832) | (1,265) | (1,269)
Deferred tax liability [Member] | Unrealised foreign exchange gains (losses) [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | 0 | (10) | (1)
Recognized in Profit or Loss | (1) | 10 | (9)
Ending balance | (1) | 0 | (10)
Deferred tax liability [Member] | Others [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (1) | (1) | (1)
Recognized in Profit or Loss | 0 | 0 | 0
Ending balance | (1) | (1) | (1)
Deferred tax liability [Member] | Land value incremental tax [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (95) | (95) | (95)
Ending balance | (95) | (95) | (95)
Deferred tax liability [Member] | Deferred revenue for award credits [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (29) | (46) | (2)
Recognized in Profit or Loss | (2) | 17 | (44)
Ending balance | (31) | (29) | (46)
Deferred tax liability [Member] | Intangible assets and goodwill [member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (39) | (40) | (43)
Recognized in Profit or Loss | 7 | 1 | 3
Ending balance | (32) | (39) | (40)
Deferred tax liability [Member] | Valuation gain or loss on financial instruments, net [Member] |  |  |
Income tax [Line Items] |  |  |
Beginning balance | (1) | (3) | (5)
Effect of Retrospective Applicationof IFRS 9 (Note 5) | 1 |  |
Recognized in Profit or Loss | 0 | (1) | 0
Recognized in Other Comprehensive Income | 0 | 3 | 2
Ending balance | $ 0 | $ (1) | $ (3)

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