Basis of Presentation - Additional Information (Detail) - USD ($) shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Restricted stock units outstanding to purchase shares included in diluted earnings per common share (in shares)	4	5	4	5
Percentage of retail postpaid connections with fixed-term service plans	15.00%		15.00%			19.00%
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of applying new accounting standards, pre-tax						$ 2,906,000,000
Cash flows from operating activities			$ (26,244,000,000)	$ (16,475,000,000)
Cash flows from investing activities			(13,136,000,000)	(12,987,000,000)
Operating income	$ 7,675,000,000	$ 6,988,000,000	21,641,000,000	21,964,000,000
Net Income	$ 5,062,000,000	3,736,000,000	$ 13,974,000,000	11,767,000,000
Sale leaseback transactions, deferred gains recognition period			9 years
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income		(200,000,000)		(700,000,000)
Net Income		$ 0		0
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows from operating activities				600,000,000
Cash flows from investing activities				$ 600,000,000
Retained earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of applying new accounting standards, net of tax					$ 3,900,000,000
Cumulative effect of applying new accounting standards, pre-tax					2,900,000,000
Retained earnings | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of applying new accounting standards, pre-tax					$ 700,000,000