Employee Benefits - Additional Information (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Benefits paid, net of participant contributions and direct subsidy receipts	$ (211,000,000)
Expected future benefit payment, next twelve months, net of direct subsidies	$ 233,000,000
Ultimate health care cost trend rate	4.50%
Remaining estimated life of plan participants	9 years
Commingled funds, redemption notice	270 days
Return on plan assets	$ 1,200,000,000	$ 1,600,000,000
Expected return on plan assets	593,000,000	618,000,000
Difference between the actual and expected returns on pension and post-retirement plan assets	618,000,000	1,000,000,000.0
Active health care benefit expenses	307,000,000	381,000,000	$ 434,000,000
Participating employees' contribution to health care plan	$ 133,000,000	148,000,000	142,000,000
Common stock included in the assets of the Defined Contribution Plan (in shares)	11
Expenses related to the 401(k) Plan	$ 101,000,000	113,000,000	93,000,000
Change in Assumptions for Defined Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 3,000,000	4,000,000	38,000,000
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	6.25%
Combined Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Funded (unfunded) status of plan	$ (1,656,000,000)	(1,724,000,000)
Employer contributions	0	0	500,000,000
Expected future benefits, next twelve months	961,000,000
One-time special termination charge	21,000,000	6,000,000	15,000,000
Benefit obligation	12,202,000,000	12,217,000,000	11,594,000,000	$ 13,064,000,000
Fair value of plan assets (liabilities)	$ 10,546,000,000	$ 10,493,000,000	$ 10,033,000,000	11,060,000,000
Expected long-term rate of return on plan assets	6.00%	6.50%	6.50%
Expected long-term rate of return on plan assets before administrative expenses	5.50%
Short term interest crediting rates	2.25%
Long term interest crediting rates	4.00%
Return on plan assets	$ 1,210,000,000	$ 1,575,000,000	$ (349,000,000)
Combined Pension Plan | Defined Benefit 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 | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Funded (unfunded) status of plan	$ (1,700,000,000)	(1,700,000,000)
Employer contributions	0
Expected return on plan assets	593,000,000	618,000,000	685,000,000
Combined Pension Plan | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Funded (unfunded) status of plan	51,000,000	51,000,000
Benefits paid by company	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	$ (33,000,000)	(18,000,000)
Post-Retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Funded (unfunded) status of plan	(3,043,000,000)	(3,024,000,000)
Employer contributions	0	0
Benefits paid by company	255,000,000	$ 300,000,000	$ 311,000,000
Expected future benefits, next twelve months	$ 238,000,000
Ultimate health care cost trend rate	4.50%	4.50%	4.50%
Benefit obligation	$ 3,048,000,000	$ 3,037,000,000	$ 2,977,000,000	$ 3,375,000,000
Fair value of plan assets (liabilities)	$ 5,000,000	$ 13,000,000	$ 18,000,000
Expected long-term rate of return on plan assets	4.00%	4.00%	4.00%
Expected return on plan assets	$ 1,000,000	$ 1,000,000	$ 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	6.25%	6.50%