Summary of Significant Accounting Policies and Other Information New Accounting Pronouncements - Cumulative Effects of Accounting Standards Update on Statement of Operations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service									$ 22,857	$ 23,834	$ 25,368
Revenue, equipment sales, net									5,606	4,524	4,684
Revenue, equipment rentals, net									5,137	4,048	3,295
Revenues	$ 8,441	$ 8,601	$ 8,433	$ 8,125	$ 8,083	$ 8,239	$ 7,927	$ 8,157	33,600	32,406	33,347
Net operating expenses:
Cost of services (exclusive of depreciation and amortization included below)									6,664	6,801	7,861
Cost of equipment sales									6,082	6,109	6,583
Cost of equipment rentals									643	493	975
Selling, general and administrative									7,774	8,087	7,994
Impairments									2,000	0	0
Depreciation, network and other									4,245	3,976	3,982
Depreciation, equipment rentals									4,538	3,792	3,116
Amortization									608	812	1,052
Goodwill, Impairment Loss									2,000
Other Operating Income (Expense), Net									648	391	(20)
Costs and Expenses									33,202	29,679	31,583
Operating Income (Loss)	$ (1,674)	$ 479	$ 778	$ 815	$ 236	$ 727	$ 601	$ 1,163	398	2,727	1,764
Nonoperating Income (Expense)									(2,376)	(2,424)	(2,535)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(1,978)	303	(771)
Income Tax Expense (Benefit)									(35)	(7,074)	435
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									$ (1,943)	$ 7,377	$ (1,206)
Earnings Per Share, Basic	$ (0.53)	$ (0.03)	$ 0.05	$ 0.04	$ 0.02	$ 1.79	$ (0.01)	$ 0.05	$ (0.48)	$ 1.85	$ (0.30)
Earnings Per Share, Diluted	$ (530,000)	$ (30,000)	$ 50,000	$ 40,000	$ 20,000	$ 1,760,000	$ (10,000)	$ 50,000	$ (0.48)	$ 1.81	$ (0.30)
Weighted Average Number of Shares Outstanding, Basic									4,057	3,999	3,981
Weighted Average Number of Shares Outstanding, Diluted									4,057	4,078	3,981
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service									$ (728)
Revenue, equipment sales, net									1,326
Revenue, equipment rentals, net									(63)
Revenues									535
Net operating expenses:
Cost of services (exclusive of depreciation and amortization included below)									(78)
Cost of equipment sales									145
Cost of equipment rentals									0
Selling, general and administrative									(390)
Depreciation, network and other									0
Depreciation, equipment rentals									0
Amortization									0
Goodwill, Impairment Loss									0
Other Operating Income (Expense), Net									0
Costs and Expenses									(323)
Operating Income (Loss)									858
Nonoperating Income (Expense)									0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									858
Income Tax Expense (Benefit)									(180)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									$ 678
Earnings Per Share, Basic									$ 0
Earnings Per Share, Diluted									$ 0
Weighted Average Number of Shares Outstanding, Basic									0
Weighted Average Number of Shares Outstanding, Diluted									0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service									$ 23,585
Revenue, equipment sales, net									4,280
Revenue, equipment rentals, net									5,200
Revenues									33,065
Net operating expenses:
Cost of services (exclusive of depreciation and amortization included below)									6,742
Cost of equipment sales									5,937
Cost of equipment rentals									643
Selling, general and administrative									8,164
Depreciation, network and other									4,245
Depreciation, equipment rentals									4,538
Amortization									608
Goodwill, Impairment Loss									2,000
Other Operating Income (Expense), Net									648
Costs and Expenses									33,525
Operating Income (Loss)									(460)
Nonoperating Income (Expense)									(2,376)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(2,836)
Income Tax Expense (Benefit)									215
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									$ (2,621)
Earnings Per Share, Basic									$ 0
Earnings Per Share, Diluted									$ 0
Weighted Average Number of Shares Outstanding, Basic									4,057
Weighted Average Number of Shares Outstanding, Diluted									4,057