New Accounting Pronouncements - Cumulative Effects of Accounting Standards Updates on Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Apr. 01, 2018	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	$ 3,555	$ 3,808	$ 3,711
Device and accessory inventory	859	979	1,003
Prepaid expenses and other current assets	1,121	846	575
Costs to acquire a customer contract	1,379	1,219
Other assets	1,163	964	921
Accrued expenses and other current liabilities	3,370	3,927	3,962
Deferred tax liabilities	7,704	7,660	7,294
Other liabilities	3,428	3,451	3,483
(Accumulated deficit) retained earnings	432	60	(1,255)
Accumulated other comprehensive loss	(308)	(321)	(313)
Balances without adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	3,470
Device and accessory inventory	881
Prepaid expenses and other current assets	691
Costs to acquire a customer contract	0
Other assets	1,004
Accrued expenses and other current liabilities	3,397
Deferred tax liabilities	7,251
Other liabilities	3,460
(Accumulated deficit) retained earnings	(1,205)
Scenario, Previously Reported
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
Costs to acquire a customer contract			0
Other assets			921
Accrued expenses and other current liabilities			3,962
Deferred tax liabilities			7,294
Other liabilities			3,483
(Accumulated deficit) retained earnings			(1,255)
Accumulated other comprehensive loss			$ (313)
Accounting Standards Update 2014-09 | Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	85
Device and accessory inventory	(22)
Prepaid expenses and other current assets	430
Costs to acquire a customer contract	1,379
Other assets	159
Accrued expenses and other current liabilities	(27)
Deferred tax liabilities	453
Other liabilities	(32)
(Accumulated deficit) retained earnings	$ 1,637
Accounting Standards Update 2014-09 | Restatement Adjustment
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
Costs to acquire a customer contract		1,219
Other assets		43
Accrued expenses and other current liabilities		(35)
Deferred tax liabilities		366
Other liabilities		(32)
(Accumulated deficit) retained earnings		1,307
Accounting Standards Update, Other | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
(Accumulated deficit) retained earnings		8
Accumulated other comprehensive loss		$ (8)