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Operating Expenses - Summary of Operating Expenses (Detail) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Profit (loss) [abstract]      
Salaries and wages ₩ 4,123,680 ₩ 3,974,233 ₩ 3,845,842
Depreciation 2,605,128 2,530,252 2,674,205
Depreciation of right-of-use assets 404,175 442,629  
Amortization of intangible assets 624,982 656,611 607,527
Commissions 965,461 1,115,477 1,080,168
Interconnection charges 500,081 534,025 579,613
International interconnection fee 172,529 240,254 226,627
Purchase of inventories 3,681,801 4,453,820 4,414,094
Changes of inventories 257,041 282,957 (432,607)
Sales commission 2,337,127 2,315,731 1,942,841
Service cost 2,102,875 1,610,261 1,540,869
Utilities 360,797 332,816 323,411
Taxes and dues 283,197 277,742 285,131
Rent 136,355 193,357 460,377
Insurance premium 71,018 82,404 73,654
Installation fee 132,117 155,178 143,669
Advertising expenses 132,466 150,166 157,675
Research and development expenses 156,940 165,028 176,758
Card service cost 2,941,669 3,066,766 3,112,618
Others 1,428,875 1,292,512 1,122,718
Total ₩ 23,418,314 ₩ 23,872,219 ₩ 22,335,190