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Segment Information - Other Segment Information (Detail) $ in Millions, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 TWD ($)
Disclosure of operating segments [Line Items] |  |  |  |
Share of the profits of associates and joint ventures accounted for using equity method | $ 419 | $ 14 | $ 515 | $ 897
Interest income | 205 | 7 | 189 | 306
Interest expenses | 22 | 1 | 20 | 33
Operating costs and expenses | 180,706 |  | 181,390 | 181,329
Depreciation and amortization | 31,930 |  | 32,485 | 33,448
Capital expenditure | 26,875 | 907 | 23,517 | 25,084
Impairment loss on property, plant and equipment | 0 | 0 | 596 | 138
Reversal of impairment loss | 11 | $ 0 | 148 | 142
Domestic Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Interest income | 21 |  | 15 | 19
Operating costs and expenses | 62,795 |  | 64,230 | 64,960
Depreciation and amortization | 15,614 |  | 16,414 | 17,487
Capital expenditure | 11,647 |  | 9,846 | 10,196
Impairment loss on property, plant and equipment | 0 |  | 0 | 22
Reversal of impairment loss | 11 |  | 148 | 142
Mobile Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Interest income | 15 |  | 11 | 19
Interest expenses | 6 |  | 2 | 10
Operating costs and expenses | 80,275 |  | 79,593 | 81,213
Depreciation and amortization | 11,001 |  | 10,620 | 10,444
Capital expenditure | 9,742 |  | 8,981 | 8,596
Impairment loss on property, plant and equipment | 0 |  | 596 | 116
Internet Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Interest income | 9 |  | 7 | 11
Operating costs and expenses | 13,288 |  | 13,160 | 12,062
Depreciation and amortization | 3,385 |  | 3,626 | 3,611
Capital expenditure | 2,779 |  | 2,718 | 4,795
International Fixed Communications Business [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Interest income | 15 |  | 6 | 2
Operating costs and expenses | 13,385 |  | 14,313 | 14,411
Depreciation and amortization | 1,477 |  | 1,451 | 1,536
Capital expenditure | 1,580 |  | 1,136 | 968
Other [Member] |  |  |  |
Disclosure of operating segments [Line Items] |  |  |  |
Share of the profits of associates and joint ventures accounted for using equity method | 419 |  | 515 | 897
Interest income | 145 |  | 150 | 255
Interest expenses | 16 |  | 18 | 23
Operating costs and expenses | 10,963 |  | 10,094 | 8,683
Depreciation and amortization | 453 |  | 374 | 370
Capital expenditure | $ 1,127 |  | $ 836 | $ 529

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