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Subsidiaries - Detailed Information of Equity Transactions (Detail) - TWD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Chunghwa Investment Co Ltd Disposed Some Shares Of Chunghwa Precision Test Tech Co Ltd [Member] |  |  |
Disclosure of classes of share capital [Line Items] |  |  |
Cash consideration received from (paid to)Noncontrolling interests |  | $ 83.0 | $ 45.0
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests |  | (25.0) | (18.0)
Differences arising from equity transactions |  | 58.0 | 27.0
Additional paid-in capital - difference between consideration received or paid and the carrying amount of the subsidiaries’ net assets upon actual disposal or acquisition |  | 58.0 | 27.0
Senao International Co Ltd Purchased Its Treasury Stock [Member] |  |  |
Disclosure of classes of share capital [Line Items] |  |  |
Cash consideration received from (paid to)Noncontrolling interests |  |  | (492.0)
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests |  |  | 416.0
Differences arising from equity transactions |  |  | (76.0)
Additional paid-in capital - arising from changes in equities of subsidiaries |  |  | (15.0)
Unappropriated earnings |  |  | (61.0)
Senao International Co Ltd Participated In Youths Share Subscription [Member] |  |  |
Disclosure of classes of share capital [Line Items] |  |  |
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests |  |  | (0.4)
Differences arising from equity transactions |  |  | (0.4)
Additional paid-in capital - arising from changes in equities of subsidiaries |  |  | $ (0.4)
Chunghwa Investment Co Ltd Did Not Participate In The Capital Increase Of Chunghwa Precision Test Tech Co Ltd [Member] |  |  |
Disclosure of classes of share capital [Line Items] |  |  |
Cash consideration received from (paid to)Noncontrolling interests | $ 2,552.0 | 1,175.0 |
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests | (1,750.0) | (786.0) |
Differences arising from equity transactions | 802.0 | 389.0 |
Additional paid-in capital - arising from changes in equities of subsidiaries | 802.0 | $ 389.0 |
Senao International Co Ltd Transferred Its Treasury Stock [Member] |  |  |
Disclosure of classes of share capital [Line Items] |  |  |
Cash consideration received from (paid to)Noncontrolling interests | 164.0 |  |
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests | (137.0) |  |
Differences arising from equity transactions | 27.0 |  |
Additional paid-in capital - arising from changes in equities of subsidiaries | 27.0 |  |
Chunghwa Telecom Co Ltd and Chunghwa Investment Co Ltd Disposed Some Shares of CHIEF Telecom Inc [Member] |  |  |
Disclosure of classes of share capital [Line Items] |  |  |
Cash consideration received from (paid to)Noncontrolling interests | 106.0 |  |
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests | (29.0) |  |
Differences arising from equity transactions | 77.0 |  |
Additional paid-in capital - difference between consideration received or paid and the carrying amount of the subsidiaries’ net assets upon actual disposal or acquisition | $ 77.0 |  |

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- Definition Difference between consideration received or paid and carrying amount of subsidiaries' net assets upon actual disposal or acquisition effect on additional paid-in capital. + References No definition available. + Details
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- Definition Proportionate share of the carrying amount of the net assets of the subsidiary transferred to from noncontrolling interests. + References No definition available. + Details
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