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Deferred Income Tax and Income Tax Expense - Summary of Movement in Deferred Income Tax Assets and Liabilities (Detail) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning ₩ (612) ₩ 260,584  
Consolidated statements of operations (72,124) (199,527) ₩ 15,016
Other comprehensive income 77,103 (61,669)  
Ending 4,367 (612) 260,584
Deferred tax liabilities derivative assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (10,898)    
Consolidated statements of operations 10,055 (10,250)  
Other comprehensive income   (648)  
Ending (843) (10,898)  
Deferred tax liabilities investment in subsidiaries associates and joint ventures [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (108,191) (93,604)  
Consolidated statements of operations (64,553) (14,622)  
Other comprehensive income (8,820) 35  
Ending (181,564) (108,191) (93,604)
Deferred tax liabilities depreciation [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (11,606) (424)  
Consolidated statements of operations 7,431 (11,182)  
Ending (4,175) (11,606) (424)
Deferred tax liabilities advanced depreciation provision [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (313,121) (313,184)  
Consolidated statements of operations 1,203 63  
Ending (311,918) (313,121) (313,184)
Deferred tax liabilities deposits for severance benefits [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (496,853) (398,982)  
Consolidated statements of operations (26,419) (99,126)  
Other comprehensive income 2,015 1,255  
Ending (521,257) (496,853) (398,982)
Deferred tax liabilities accrued income [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (1,541) (1,558)  
Consolidated statements of operations (212) 17  
Ending (1,753) (1,541) (1,558)
Deferred tax liabilities reserve for technology and human resource development [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (204) (204)  
Ending (204) (204) (204)
Other deferred tax liabilities [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (523,150) (284,255)  
Consolidated statements of operations 53,797 (238,725)  
Other comprehensive income 0 (170)  
Ending (469,353) (523,150) (284,255)
Deferred tax derivative assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning   8,341  
Consolidated statements of operations   (850)  
Other comprehensive income   (7,491)  
Ending     8,341
Deferred tax assets provision for impairment or trade receivables [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 84,071 99,887  
Consolidated statements of operations 4,994 (15,816)  
Ending 89,065 84,071 99,887
Deferred tax assets inventory valuation [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 23 121  
Consolidated statements of operations (259) (98)  
Ending (236) 23 121
Deferred tax assets contribution for construction [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 16,154 16,800  
Consolidated statements of operations 246 (646)  
Ending 16,400 16,154 16,800
Deferred Tax Assets Provisions [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 32,824 36,178  
Consolidated statements of operations 1,198 (3,354)  
Ending 34,022 32,824 36,178
Deferred tax assets property, plant and equipment [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 228,655 230,278  
Consolidated statements of operations (1,695) (1,623)  
Ending 226,960 228,655 230,278
Deferred Tax assets assets retirement obligation [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 29,016 24,532  
Consolidated statements of operations (883) 4,484  
Ending 28,133 29,016 24,532
Deferred tax assets withholding of facilities expenses [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 6,183 6,609  
Consolidated statements of operations (436) (426)  
Ending 5,747 6,183 6,609
Deferred tax assets deduction of installment receivables [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 48 42  
Consolidated statements of operations (20) 6  
Ending 28 48 42
Deferred tax assets gain or loss foreign currency translation [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 20,677 10,672  
Consolidated statements of operations (20,539) 10,005  
Ending 138 20,677 10,672
Deferred tax assets deferred revenue [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 35,800 39,641  
Consolidated statements of operations 7,230 (3,841)  
Ending 43,030 35,800 39,641
Deferred tax assets real-estate sales [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 13,685 21,728  
Consolidated statements of operations (13,685) (8,043)  
Ending   13,685 21,728
Other deferred tax assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 708,437 537,209  
Consolidated statements of operations (123,798) 174,177  
Other comprehensive income 948 (2,949)  
Ending 585,587 708,437 537,209
Deferred tax assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 1,905,480 1,673,777  
Consolidated statements of operations (20,492) 235,361  
Other comprehensive income 6,274 (3,658)  
Ending 1,891,262 1,905,480 1,673,777
Deferred tax assets defined benefit liabilities [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 569,471 513,842  
Consolidated statements of operations 13,707 48,847  
Other comprehensive income 15,186 6,782  
Ending 598,364 569,471 513,842
Deferred tax assets tax credit carry forwards [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 128,245 102,898  
Consolidated statements of operations 16,686 25,347  
Ending 144,931 128,245 102,898
Deferred tax liabilities contract assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (53,750) (11,505)  
Consolidated statements of operations (112,794) (42,245)  
Ending (166,544) (53,750) (11,505)
Deferred tax liabilities financial assets at fair value through profit or loss [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (323) (661)  
Consolidated statements of operations (304) 338  
Ending (627) (323) (661)
Deferred tax liabilities Financial assets at fair value through other comprehensive income [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (103,837) (41,798)  
Consolidated statements of operations (4,420) (3,556)  
Other comprehensive income 77,634 (58,483)  
Ending (30,623) (103,837) (41,798)
Deferred tax assets unsettled expenses [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning 160,436 127,897  
Consolidated statements of operations (24,358) 32,539  
Ending 136,078 160,436 127,897
Temporary differences [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (128,857) 157,686  
Consolidated statements of operations (88,810) (224,874)  
Other comprehensive income 77,103 (61,669)  
Ending (140,564) (128,857) 157,686
Deferred tax liabilities [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (2,034,337) (1,516,091)  
Consolidated statements of operations (68,318) (460,235)  
Other comprehensive income 70,829 (58,011)  
Ending (2,031,826) (2,034,337) (1,516,091)
Deferred Tax Liabilities Contract Cost [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Beginning (410,863) (369,916)  
Consolidated statements of operations 67,898 (40,947)  
Ending (342,965) ₩ (410,863) ₩ (369,916)
Deferred Tax Assets Derivative instrument [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Consolidated statements of operations 40,342    
Other comprehensive income (9,860)    
Ending 30,482    
Deferred Tax Assets Contract Assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]      
Consolidated statements of operations 97,464    
Ending ₩ 97,464