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Cash Generated from Operations - Summary of Cash Flows from Operating Activities (Detail) - KRW (₩) ₩ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Cash flows from (used in) operating activities [abstract] |  |  |
Profit for the year | ₩ 699,274 | ₩ 719,412 | ₩ 546,341
Adjustments to reconcile net income |  |  |
Income tax expense | 320,060 | 314,565 | 270,656
Interest income | (303,722) | (265,817) | (108,639)
Interest expense | 268,773 | 296,894 | 302,464
Dividends income | (3,408) | (2,910) | (4,785)
Depreciation | 2,567,754 | 2,735,413 | 2,802,531
Amortization of intangible assets | 660,705 | 629,526 | 635,150
Depreciation of right-of-use assets | 452,056 |  |
Provision for severance benefits | 256,525 | 245,926 | 218,966
Impairment losses on trade receivables | 60,193 | 113,064 | 45,704
Share of net profit or loss of associates and joint ventures | 3,252 | 5,912 | 15,480
Loss(gain) on disposal of associates and joint ventures | 30 | (3,737) | 979
Gain on disposal of right-of-use assets | (1,853) |  |
Impairment loss of associates and joint ventures | 0 |  | 3,662
Impairment losses on assets held for sale | 7,586 |  |
Loss on disposal of property and equipment and investment in properties | 49,284 | 68,688 | 150,293
Loss(gain) on disposal of intangible assets | (1,248) | (4,256) | 4,271
Loss on impairment of intangible assets | 61,899 | 12,997 | 116,095
Loss on foreign currency translation | 75,998 | 68,952 | (213,341)
Loss(gain) on valuation and settlement of derivatives, net | (70,482) | (92,210) | 268,094
Gain on disposal of financial assets at fair value through profit or loss | (5,115) | (1,712) |
Gain on valuation of financial assets at fair value through profit or loss | (4,335) | (10,768) |
Gain on disposal of financial assets at amortized cost | 43 | (44) |
Impairment losses on available-for-sale financial assets | 0 |  | 9
Gain on disposal of available-for-sale financial assets | 0 |  | (89,598)
Others | 134,848 | (55,969) | (251,193)
Changes in operating assets and liabilities |  |  |
Decrease(increase) in trade receivables | (433,292) | (81,217) | (303,340)
Decrease(increase) in other receivables | (79,130) | 356,643 | (346,013)
Decrease(increase) in other current assets | 984 | (123,258) | 11,792
Decrease(increase) in other non-current assets | (178,180) | 19,556 | (43,790)
Decrease(increase) in inventories | 240,488 | (480,543) | (205,403)
Increase(decrease) in trade payables | 44,354 | (167,841) | 162,110
Increase(decrease) in other payables | (102,375) | (448,301) | 214,689
Increase in other current liabilities | 43,384 | 291,548 | 288,553
Increase(decrease) in other non-current liabilities | (199,547) | 144,072 | 174,618
Decrease(Increase) in provisions | (12,164) | 85,946 | (12,574)
Decrease(Increase) in deferred revenue | 641 | 48,201 | (13,086)
Increase in plan assets | (375,499) | (53,301) | (203,420)
Payment of severance benefits | (119,716) | (153,209) | (118,391)
Cash generated from operations (1+2+3) | ₩ 4,058,065 | ₩ 4,212,222 | ₩ 4,318,884

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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Adjustments for payments of severance benefits. + References No definition available. + Details
Name: | kt_AdjustmentsForPaymentsOfSeveranceBenefits
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Adjustments for provision for severance benefits. + References No definition available. + Details
Name: | kt_AdjustmentsForProvisionForSeveranceBenefits
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Adjustments for share of net profit of associates and joint ventures. + References No definition available. + Details
Name: | kt_AdjustmentsForShareOfNetProfitOfAssociatesAndJointVentures
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Change in Operating Assets and Liabilities [abstract] + References No definition available. + Details
Name: | kt_ChangeInOperatingAssetsAndLiabilitiesAbstract
Namespace Prefix: | kt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Gain (loss ) valuation of financial assets fair value of profit or loss. + References No definition available. + Details
Name: | kt_GainLossValuationOfFinancialAssetsFairValueOfProfitOrLoss
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Gain on disposal of right-of-use assets. + References No definition available. + Details
Name: | kt_GainOnDisposalOfRightofuseAssets
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Impairment losses on current assets held for sale. + References No definition available. + Details
Name: | kt_ImpairmentLossesOnCurrentAssetsHeldForSale
Namespace Prefix: | kt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration