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Summary of Significant Accounting Policies - Detail Information of Subsidiaries (Detail) | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Mar. 31, 2018 | Nov. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 01, 2015
Light Era Development Co., Ltd. (LED) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Planning and development of real estate and intelligent buildings, and property management |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Donghwa Telecom Co., Ltd. (DHT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International private leased circuit, IP VPN service, and IP transit services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Telecom Singapore Pte., Ltd. (CHTS) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International private leased circuit, IP VPN service, and IP transit services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa System Integration Co., Ltd. (CHSI) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Providing system integration services and telecommunications equipment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Investment Co., Ltd. (CHI) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Percentage of Ownership | 89.00% |  |  |  | 89.00% |  |
CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Network integration, internet data center (“IDC”), communications integration and cloud application services |  |  |  |  |  |
Percentage of Ownership | 67.00% |  |  |  | 69.00% |  |
CHYP Multimedia Marketing & Communications Co., Ltd. (“CHYP”) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Digital information supply services and advertisement services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Prime Asia Investments Group Ltd. (B.V.I.) (Prime Asia) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Spring House Entertainment Tech. Inc. (SHE) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Digital entertainment contents production, animated character licensing and endorsement, and mobile digital platform construction |  |  |  |  |  |
Percentage of Ownership | 56.00% |  |  |  | 56.00% |  |
Chunghwa Telecom Global, Inc. ("CHTG") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International private leased circuit, internet services, and transit services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Telecom Vietnam Co., Ltd. (CHTV) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Intelligent energy saving solutions, international circuit, and information and communication technology (“ICT”) services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Smartfun Digital Co., Ltd. (SFD) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Providing diversified family education digital services |  |  |  |  |  |
Percentage of Ownership | 65.00% |  |  |  | 65.00% |  |
Chunghwa Telecom Japan Co., Ltd. (CHTJ) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International private leased circuit, IP VPN service, and IP transit services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Sochamp Technology Inc. (CHST) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Design, development and production of Automatic License Plate Recognition software and hardware |  |  |  |  |  |
Percentage of Ownership | 51.00% |  |  |  | 51.00% |  |
Honghwa International Co., Ltd. (HHI) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Telecommunication engineering, sales agent of mobile phone plan application and other business services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Leading Photonics Tech Co., Ltd. (CLPT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Production and sale of electronic components and finished products |  |  |  |  |  |
Percentage of Ownership | 75.00% |  |  |  | 75.00% |  |
Chunghwa Telecom (Thailand) Co., Ltd. (“CHTT”) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International private leased circuit, IP VPN service, ICT and cloud VAS services |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  | 100.00% |  |  |
New Prospect Investments Holdings Ltd. (B.V.I.) (New Prospect) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Percentage of Ownership |  |  |  |  | 100.00% |  |
Chunghwa Telecom Security Co Ltd [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Computing equipment installation, wholesale of computing and business machinery equipment and software, management consulting services, data processing services, digital information supply services and internet identify services |  |  |  |  |  |
Percentage of Ownership | 80.00% |  |  |  |  |  |
Chunghwa Telecom Co., Ltd. [Member] | Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Handset and peripherals retailer; sales of CHT mobile phone plans as an agent |  |  |  |  |  |
Percentage of Ownership | 28.93% |  |  |  | 29.00% |  |
Chunghwa Telecom Co., Ltd. [Member] | CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership | 70.43% | 69.31% |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  |  |  |  |  | 29.31% | 28.18%
Senao International Co., Ltd. (SENAO) [Member] | Senao International (Samoa) Holding Ltd. (SIS) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International investment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International Co., Ltd. (SENAO) [Member] | Youth Co., Ltd. (Youth) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Sale of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 89.00% |  |  |  | 89.00% |  |
Senao International Co., Ltd. (SENAO) [Member] | Aval Technologies Co., Ltd. (Aval) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Sale of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International Co., Ltd. (SENAO) [Member] | SENYOUNG Insurance Agent Co., Ltd. [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Property and liability insurance agency |  |  |  |  |  |
Percentage of Ownership | 100.00% |  | 100.00% |  |  |  |
Youth Co., Ltd. (Youth) [Member] | ISPOT Co., Ltd. (ISPOT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Sale of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Youth Co., Ltd. (Youth) [Member] | Youyi Co., Ltd. (Youyi) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Maintenance of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Unigate Telecom Inc. (Unigate) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Telecommunications and internet service |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Chief International Corp. (CIC) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Telecommunications and internet service |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Shanghai Chief Telecom Co., Ltd. (SCT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Telecommunications and internet service |  |  |  |  |  |
Percentage of Ownership | 49.00% |  |  |  | 49.00% |  |
Chunghwa System Integration Co., Ltd. (CHSI) [Member] | Concord Technology Co., Ltd. (Concord) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Spring House Entertainment Tech. Inc. (SHE) [Member] | Ceylon Innovation Co., Ltd. (CEI) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | E-book publishing and copyright negotiation of digital music |  |  |  |  |  |
Chunghwa Investment Co., Ltd. (CHI) [Member] | Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Production and sale of semiconductor testing components and printed circuit board |  |  |  |  |  |
Percentage of Ownership | 38.00% |  |  |  | 41.00% |  |
Chunghwa Investment Co., Ltd. (CHI) [Member] | Chunghwa Investment Holding Co., Ltd. (CIHC) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Concord Technology Co., Ltd. (Concord) [Member] | Glory Network System Service (Shanghai) Co., Ltd. (GNSS (Shanghai) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Design, development and production of computer and internet software, installment, maintenance and consulting services of information system integration, and sales of self-production products |  |  |  |  |  |
Percentage of Ownership |  |  |  |  | 100.00% |  |
Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] | Chunghwa Precision Test Tech. USA Corporation (CHPT (US) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Design and after-sale services of semiconductor testing components and printed circuit board |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] | CHPT Japan Co., Ltd. (CHPT (JP) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Related services of electronic parts, machinery processed products and printed circuit board |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] | Chunghwa Precision Test Tech. International Ltd. ("CHPT (International)") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Wholesale and retail of electronic materials, and investment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International (Samoa) Holding Ltd. (SIS) [Member] | Senao International HK Limited (SIHK) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | International investment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Investment Holding Co., Ltd. (CIHC) [Member] | CHI One Investment Co., Limited (COI) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Prime Asia Investments Group Ltd. (B.V.I.) (Prime Asia) [Member] | Chunghwa Hsingta Co., Ltd. ("CHC") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Investment |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International HK Limited (SIHK) [Member] | Senao Trading (Fujian) Co., Ltd. (STF) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Sale of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International HK Limited (SIHK) [Member] | Senao International Trading (Shanghai) Co., Ltd. (SITS) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Sale of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International HK Limited (SIHK) [Member] | Senao International Trading (Shanghai) Co Ltd (SEITS) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Maintenance of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Senao International HK Limited (SIHK) [Member] | Senao International Trading (Jiangsu) Co., Ltd. (SITJ) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Sale of information and communication technologies products |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Hsingta Co., Ltd. ("CHC") [Member] | Chunghwa Telecom (China) Co., Ltd. ("CTC") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Integrated information and communication solution services for enterprise clients, and intelligent energy network service |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |
Chunghwa Hsingta Co., Ltd. ("CHC") [Member] | Jiangsu Zhenhua Information Technology Company, LLC. ("JZIT") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Providing intelligent energy saving solution and intelligent buildings services |  |  |  |  |  |
Percentage of Ownership | 75.00% |  |  |  | 75.00% |  |
Chunghwa Precision Test Tech. International Ltd. ("CHPT (International)") [Member] | Shanghai Taihua Electronic Technology Limited ("STET") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Main Businesses and Products | Design of printed circuit board and related consultation service |  |  |  |  |  |
Percentage of Ownership | 100.00% |  |  |  | 100.00% |  |

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