Planned Divestiture of the Latin American and ILEC Businesses - Additional Information (Details) - USD ($) $ in Millions			3 Months Ended		9 Months Ended
	Aug. 03, 2021	Jul. 25, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciation and amortization					$ 3,142	$ 3,515
Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assumed indebtedness from disposal of business			$ 2,291		2,291
Depreciation and amortization			107		107
Disposal Group, Held-for-sale, Not Discontinued Operations | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax net income (loss)			280	$ 175	649	563
Disposal Group, Held-for-sale, Not Discontinued Operations | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax net income (loss)			343	214	793	687
Disposal Group, Held-for-sale, Not Discontinued Operations | Latin American Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assumed indebtedness from disposal of business			412		412
Disposal Group, Held-for-sale, Not Discontinued Operations | Latin American Business | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax net income (loss)			59	36	131	108
Disposal Group, Held-for-sale, Not Discontinued Operations | Latin American Business | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax net income (loss)			72	45	160	131
Disposal Group, Held-for-sale, Not Discontinued Operations | Latin American Business | Level 3 Parent, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration from disposal of business		$ 2,700
Working capital, other purchase price adjustments and transaction expenses		$ 50
Disposal Group, Held-for-sale, Not Discontinued Operations | ILEC Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration from disposal of business	$ 7,500
Assumed indebtedness from disposal of business	1,400		1,879		1,879
Purchaser's transaction expenses	$ 1,700
Disposal Group, Held-for-sale, Not Discontinued Operations | ILEC Business | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax net income (loss)			221	139	518	455
Disposal Group, Held-for-sale, Not Discontinued Operations | ILEC Business | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax net income (loss)			$ 271	$ 169	$ 633	$ 556