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Employee Benefits - Additional Information (Details) shares in Millions | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2022 USD ($) | Dec. 31, 2023 | Dec. 31, 2022 USD ($) retiree shares | Dec. 31, 2021 USD ($) shares | Dec. 31, 2020 USD ($) | Jan. 01, 2022 USD ($) Employee | Dec. 31, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Benefits paid, net of participant contributions and direct subsidy receipts |  |  | $ 210,000,000 |  |  |  |
Expected future benefit payment, next twelve months, net of direct subsidies |  |  | $ 210,000,000 |  |  |  |
Ultimate health care cost trend rate |  |  | 4.50% |  |  |  |
Commingled funds, redemption notice period |  |  | 180 days |  |  |  |
Actual (losses) gains |  |  | $ (987,000,000) | $ 422,000,000 |  |  |
Expected return on plan assets |  |  | 329,000,000 | $ 535,000,000 |  |  |
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] |  |  |  | Other income (expense), net |  |  |
Difference between the actual and expected returns on pension and post-retirement plan assets |  |  | (1,300,000,000) | $ (113,000,000) |  |  |
Active health care benefit expenses |  |  | 296,000,000 | 309,000,000 | $ 307,000,000 |  |
Participating employees' contribution to health care plan |  |  | $ 101,000,000 | $ 120,000,000 | 133,000,000 |  |
Common stock included in the assets of the Defined Contribution Plan (in shares) | shares |  |  | 10 | 10 |  |  |
Expenses related to the 401(k) Plan |  |  | $ 91,000,000 | $ 96,000,000 | 101,000,000 |  |
US |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Pension liability |  |  |  | 1,400,000,000 |  |  |
Change in Assumptions for Defined Benefit Plans |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Benefit obligation |  |  |  | 37,000,000 | (3,000,000) |  |
Minimum |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Expected health care cost trend rate |  |  | 5.00% |  |  |  |
Maximum |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Expected health care cost trend rate |  |  | 7.20% |  |  |  |
Combined Pension Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Funded (unfunded) status of plan |  |  | $ (580,000,000) | (1,147,000,000) |  |  |
Expected future benefits, next twelve months |  |  | 566,000,000 |  |  |  |
One-time special termination charge |  |  |  | 6,000,000 | 21,000,000 |  |
Settlements |  |  |  | 383,000,000 |  |  |
Benefit obligation |  |  | $ 5,295,000,000 | 9,678,000,000 | 12,202,000,000 |  | $ 12,217,000,000
Short term interest crediting rates |  |  | 3.75% |  |  |  |
Long term interest crediting rates |  |  | 3.50% |  |  |  |
Fair value of plan assets |  |  | $ 4,715,000,000 | $ 8,531,000,000 | $ 10,546,000,000 |  | 10,493,000,000
Expected long-term rate of return on plan assets |  |  | 5.50% | 5.50% | 6.50% |  |
Expected long-term rate of return on plan assets before administrative expenses |  |  | 6.50% |  |  |  |
Actual (losses) gains |  |  | $ (987,000,000) | $ 422,000,000 | $ 1,210,000,000 |  |
Combined Pension Plan | Forecast |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Expected long-term rate of return on plan assets |  | 7.00% |  |  |  |  |
Combined Pension Plan | New Lumen Pension Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Contributions | $ 319,000,000 |  |  |  |  |  |
Benefit obligation |  |  |  |  |  | $ 2,500,000,000 |
Fair value of plan assets |  |  |  |  |  | $ 2,200,000,000 |
Number of active participants (in employees) | Employee |  |  |  |  |  | 2,500 |
Number of other participants (in employees) | Employee |  |  |  |  |  | 19,000 |
Combined Pension Plan | Debt Security |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Plan assets, target allocation, percentage |  |  | 55.00% |  |  |  |
Combined Pension Plan | Derivatives |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Plan assets, target allocation, percentage |  |  | 45.00% |  |  |  |
Combined Pension Plan | US |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Number of retirees | retiree |  |  | 22,600 |  |  |  |
Combined Pension Plan | Qualified Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Funded (unfunded) status of plan |  |  | $ (580,000,000) | (1,100,000,000) |  |  |
Contributions |  |  | 0 | 0 |  |  |
Settlements |  |  | 0 | 383,000,000 | 0 |  |
Expected return on plan assets |  |  | 385,000,000 | 535,000,000 | 593,000,000 |  |
Combined Pension Plan | Nonqualified Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Funded (unfunded) status of plan |  |  | (35,000,000) | (46,000,000) |  |  |
Benefits paid by company |  |  | 5,000,000 | 5,000,000 |  |  |
Expected future benefits, next twelve months |  |  | $ 5,000,000 |  |  |  |
Combined Pension Plan | Level 3 Parent, LLC |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Amortization period of the plan shortfall |  |  | 7 years |  |  |  |
Combined Pension Plan | Legacy Level 3 |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Funded (unfunded) status of plan |  |  |  | (17,000,000) |  |  |
Post-Retirement Benefit Plans |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Funded (unfunded) status of plan |  |  | $ (1,990,000,000) | (2,776,000,000) |  |  |
Contributions |  |  | 0 | 0 |  |  |
Benefits paid by company |  |  | 249,000,000 | $ 247,000,000 | $ 255,000,000 |  |
Expected future benefits, next twelve months |  |  | $ 213,000,000 |  |  |  |
Ultimate health care cost trend rate |  |  | 4.50% | 4.50% | 4.50% |  |
Benefit obligation |  |  | $ 1,995,000,000 | $ 2,781,000,000 | $ 3,048,000,000 |  | $ 3,037,000,000
Fair value of plan assets |  |  | $ 5,000,000 | $ 5,000,000 | $ 5,000,000 |  |
Expected long-term rate of return on plan assets |  |  | 4.00% | 4.00% | 4.00% |  |
Expected return on plan assets |  |  | $ 0 | $ 0 | $ 1,000,000 |  |
Post-Retirement Benefit Plans | Minimum |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Expected health care cost trend rate |  |  | 5.00% | 5.00% |  |  |
Post-Retirement Benefit Plans | Maximum |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Expected health care cost trend rate |  |  | 7.20% | 5.75% |  |  |

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- Definition Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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- Definition Indicates line item in statement of income or comprehensive income that includes expected return (loss) on plan asset component of net periodic benefit (cost) credit for defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI https://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3A -URI https://asc.fasb.org/extlink&oid=123453770&loc=SL108413299-114919 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(7) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920 + Details
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- Definition Ultimate trend rate for health care cost for defined benefit postretirement plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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