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Related Party Transactions - Significant Financial Transactions Between the Company, Excluding the Controlling Company, and Related Parties (Detail) - KRW (₩)
₩ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Disclosure of transactions between related parties [line items]      
Beginning ₩ 581,267 ₩ 510,570 ₩ 532,515
Lend 78,628 78,487 245,810
Collect (100,546) (70,961) (280,605)
Others 13,219 63,171 12,850
Ending 572,568 581,267 510,570
UITrans LRT Co., Ltd. [member]      
Disclosure of transactions between related parties [line items]      
Beginning 51,051 46,335 36,510
Lend 0 4,718 9,825
Collect (11,718)   0
Others 0 (2) 0
Ending 39,333 51,051 46,335
PT. Tanggamus Electric Power [member]      
Disclosure of transactions between related parties [line items]      
Beginning 3,854 4,826 5,014
Collect (3,089) (1,288) (270)
Others 1,823 316 82
Ending 2,588 3,854 4,826
PT. Wampu Electric Power [member]      
Disclosure of transactions between related parties [line items]      
Beginning   0 3,988
Collect     (4,063)
Others     75
Ending     0
Nickel mining company SAS [member]      
Disclosure of transactions between related parties [line items]      
Beginning 68,793 64,197 63,024
Collect   0 (2,322)
Others 7,064 4,596 3,495
Ending 75,857 68,793 64,197
KRAKATAU POS-CHEM DONG-SUH CHEMICAL [member]      
Disclosure of transactions between related parties [line items]      
Beginning 1,470 2,579 3,802
Collect (1,435) (1,379) (1,323)
Others (35) 270 100
Ending 0 1,470 2,579
POS-SeAH Steel Wire (Thailand) Co., Ltd. [member]      
Disclosure of transactions between related parties [line items]      
Beginning   0 7,604
Collect     (7,828)
Others     224
Ending     0
AMCI (WA) PTY LTD [member]      
Disclosure of transactions between related parties [line items]      
Beginning 142,767 112,177 104,281
Lend 73 6,499 6,620
Collect (13,657)    
Others 7,397 24,091 1,276
Ending 136,580 142,767 112,177
POS-AUSTEM WUHAN AUTOMOTIVE CO.,LTD [member]      
Disclosure of transactions between related parties [line items]      
Beginning 5,636 5,158 10,138
Collect (5,733) 0 (5,321)
Others 97 478 341
Ending 0 5,636 5,158
Hyo-chun Co., Ltd. [member]      
Disclosure of transactions between related parties [line items]      
Beginning 2,382 2,382 2,382
Ending 2,382 2,382 2,382
POS-AUSTEM Suzhou Automotive Co., Ltd [member]      
Disclosure of transactions between related parties [line items]      
Beginning   0 12,673
Collect     (13,219)
Others     546
Ending     0
FQM Australia Holdings Pty Ltd [Member]      
Disclosure of transactions between related parties [line items]      
Beginning 292,764 256,938 247,868
Lend 0 3,126 2,637
Others (173) 32,700 6,433
Ending 292,591 292,764 256,938
POHANG E&E Co., LTD      
Disclosure of transactions between related parties [line items]      
Beginning 3,228 1,646 100
Lend 1,444 1,582 1,546
Ending 4,672 3,228 1,646
POSCO(Guangdong) Automotive Steel Co., Ltd. [member]      
Disclosure of transactions between related parties [line items]      
Beginning 6,162 14,332 35,131
Lend 64,858 59,402 225,182
Collect (64,914) (68,294) (246,259)
Others 106 722 278
Ending 6,212 6,162 ₩ 14,332
Gale International Korea, LLC [member]      
Disclosure of transactions between related parties [line items]      
Beginning 100    
Lend 344 100  
Ending 444 100  
P&O Chemical Co., Ltd      
Disclosure of transactions between related parties [line items]      
Beginning 3,060    
Lend 0 3,060  
Others (3,060)    
Ending 0 ₩ 3,060  
POS-AUSTEM Suzhou Automotive      
Disclosure of transactions between related parties [line items]      
Lend 6,143    
Ending 6,143    
CAML      
Disclosure of transactions between related parties [line items]      
Lend 5,766    
Ending ₩ 5,766