Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal
Current		$ 82	$ 335	$ 28
Deferred		(988)	5	329
State
Current		21	27	40
Deferred		16	8	21
Foreign
Current		22	26	16
Deferred		(2)	(7)	4
Total income tax expense		(849)	394	438
Income tax expense allocation
Attributable to income		(849)	394	438
Stockholders' equity:
Compensation expense for tax purposes in excess of amounts recognized for financial reporting purposes		0	(2)	(5)
Tax effect of the change in accumulated other comprehensive loss		$ 81	$ (109)	$ 59
Reconciliation of the statutory federal income tax rate to effective income tax rate
Statutory federal income tax rate (as a percent)	35.00%	35.00%	35.00%	35.00%
Federal income tax rate after enactment of Tax Reform (percentage)	21.00%
State income taxes, net of federal income tax benefit (as a percent)		3.90%	2.30%	2.60%
Change in liability for unrecognized tax position (as a percent)		1.00%	0.20%	0.40%
Tax reform (as a percent)		(209.80%)	(0.00%)	(0.00%)
Net foreign income taxes (as a percent)		(0.70%)	0.10%	0.70%
Foreign dividend paid to a domestic parent company (as a percent)		0.20%	1.80%	0.00%
Affiliate debt rationalization, (as a percent)		(0.00%)	(0.00%)	(2.60%)
Research and development credits (as a percent)		(1.40%)	(0.60%)	(2.10%)
Tax impact on sale of data centers and colocation business, (as a percent)		5.00%	0.00%	0.00%
Level 3 acquisition transaction costs, (as a percent)		6.00%	0.00%	0.00%
Other, net (as a percent)		3.60%	(0.20%)	(0.70%)
Effective income tax rate (as a percent)		(157.20%)	38.60%	33.30%
Provisional tax benefit, amount	$ 1,100
Tax expense related to sale of colocation business		$ 27
Transaction costs		32
Foreign dividend paid to a domestic parent company			$ 18
Affiliate debt rationalization				$ 34
Research and development credits				28
Changes affecting state income taxes				16
Deferred tax assets
Post-retirement and pension benefit costs	1,321	1,321	2,175
Net operating loss carryforwards	3,951	3,951	473
Other employee benefits	112	112	125
Other	714	714	342
Gross deferred tax assets	6,098	6,098	3,115
Less valuation allowance	(1,341)	(1,341)	(375)
Net deferred tax assets	4,757	4,757	2,740
Deferred tax liabilities
Property, plant and equipment, primarily due to depreciation differences	(2,935)	(2,935)	(3,626)
Goodwill and other intangible assets	(3,785)	(3,785)	(2,577)
Other	(16)	(16)	0
Gross deferred tax liabilities	(6,736)	(6,736)	(6,203)
Net deferred tax liabilities	1,979	1,979	3,463
Long-term deferred tax liability, net	2,413	2,413	3,471
Noncurrent deferred tax asset, net	434	434	8
Summary of reconciliation of the change in gross unrecognized tax benefits activity
Unrecognized tax benefits at beginning of year		16	15
Assumed in the acquisition of Level 3		18	0
Tax position of prior periods netted against deferred tax assets assumed in the acquisition of Level 3		2	0
Increase in tax positions taken in the current year		1	1
Increase in tax positions taken in the prior year		3	0
Unrecognized tax benefits at end of year	40	40	16	$ 15
Unrecognized tax benefits that would impact effective tax rate	66	66	34
Interest on income taxes accrued	56	56	$ 35
Significant change in unrecorded benefit	16	16
Federal
NOLs
Operating loss carryforward	9,100	9,100
State
NOLs
Operating loss carryforward	21,000	21,000
Foreign
NOLs
Operating loss carryforward	$ 5,800	$ 5,800