New Accounting Pronouncements - Cumulative Effects of Accounting Standards Updates on Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2018	Apr. 01, 2018	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	$ 3,455	$ 3,808	$ 3,711
Device and accessory inventory	919	979	1,003
Prepaid expenses and other current assets	1,199	846	575
Costs to acquire a customer contract	1,497	1,219
Other assets	1,128	964	921
Accrued expenses and other current liabilities	3,467	3,927	3,962
Deferred tax liabilities	7,684	7,660	7,294
Other liabilities	3,403	3,451	3,483
(Accumulated deficit) retained earnings	291	60	(1,255)
Accumulated other comprehensive loss	(333)	(321)	(313)
Balances without adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	3,356
Device and accessory inventory	941
Prepaid expenses and other current assets	672
Costs to acquire a customer contract	0
Other assets	939
Accrued expenses and other current liabilities	3,489
Deferred tax liabilities	7,177
Other liabilities	3,437
(Accumulated deficit) retained earnings	(1,548)
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	99
Device and accessory inventory	(22)
Prepaid expenses and other current assets	527
Costs to acquire a customer contract	1,497
Other assets	189
Accrued expenses and other current liabilities	(22)
Deferred tax liabilities	507
Other liabilities	(34)
(Accumulated deficit) retained earnings	$ 1,839
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)