Summary of Significant Accounting Policies and Other Information New Accounting Pronouncements - Cumulative Effects of Accounting Standards Updates on Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2019	Apr. 01, 2018	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	$ 3,554	$ 3,808	$ 3,711
Device and accessory inventory	999	979	1,003
Prepaid expenses and other current assets	1,289	846	575
Capitalized Contract Cost, Net	1,559	1,219
Other assets	1,118	964	921
Accrued expenses and other current liabilities	3,597	3,927	3,962
Deferred tax liabilities	7,556	7,660	7,294
Other liabilities	3,437	3,451	3,483
Retained Earnings (Accumulated Deficit)	(1,883)	60	(1,255)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(392)	(321)	(313)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	3,443
Device and accessory inventory	1,020
Prepaid expenses and other current assets	651
Capitalized Contract Cost, Net	0
Other assets	916
Accrued expenses and other current liabilities	3,610
Deferred tax liabilities	7,010
Other liabilities	3,466
Retained Earnings (Accumulated Deficit)	(3,868)
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]
Accounts and notes receivable, net	111
Device and accessory inventory	(21)
Prepaid expenses and other current assets	638
Capitalized Contract Cost, Net	1,559
Other assets	202
Accrued expenses and other current liabilities	(13)
Deferred tax liabilities	546
Other liabilities	(29)
Retained Earnings (Accumulated Deficit)	$ 1,985
Restatement Adjustment [Member] | Accounting standards update, other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net		0
Device and accessory inventory		0
Prepaid expenses and other current assets		0
Capitalized Contract Cost, Net		0
Other assets		0
Accrued expenses and other current liabilities		0
Deferred tax liabilities		0
Other liabilities		0
Retained Earnings (Accumulated Deficit)		8
Accumulated Other Comprehensive Income (Loss), Net of Tax		(8)
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net		97
Device and accessory inventory		(24)
Prepaid expenses and other current assets		271
Capitalized Contract Cost, Net		1,219
Other assets		43
Accrued expenses and other current liabilities		(35)
Deferred tax liabilities		366
Other liabilities		(32)
Retained Earnings (Accumulated Deficit)		1,307
Accumulated Other Comprehensive Income (Loss), Net of Tax		$ 0
Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net			3,711
Device and accessory inventory			1,003
Prepaid expenses and other current assets			575
Capitalized Contract Cost, Net			0
Other assets			921
Accrued expenses and other current liabilities			3,962
Deferred tax liabilities			7,294
Other liabilities			3,483
Retained Earnings (Accumulated Deficit)			(1,255)
Accumulated Other Comprehensive Income (Loss), Net of Tax			$ (313)