New Accounting Pronouncements - Cumulative Effects of Accounting Standards Updates on Balance Sheet (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2018	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Investing Activities	$ (4,369)	$ (953)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	4,425	$ 2,550		$ 6,659	$ 2,942
Accounts and notes receivable, net	3,492		$ 3,808	3,711
Device and accessory inventory	622		979	1,003
Prepaid expenses and other current assets	895		846	575
Costs to acquire a customer contract	1,294		1,219
Other assets	1,023		964	921
Accrued expenses and other current liabilities	3,658		3,927	3,962
Deferred tax liabilities	7,704		7,660	7,294
Other liabilities	3,382		3,451	3,483
(Accumulated deficit) retained earnings	236		60	(1,255)
Accumulated other comprehensive loss	(317)		(321)	(313)
Balances without adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts and notes receivable, net	3,404
Device and accessory inventory	644
Prepaid expenses and other current assets	557
Costs to acquire a customer contract	0
Other assets	919
Accrued expenses and other current liabilities	3,690
Deferred tax liabilities	7,298
Other liabilities	3,413
(Accumulated deficit) retained earnings	(1,223)
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	88
Device and accessory inventory	(22)
Prepaid expenses and other current assets	338
Costs to acquire a customer contract	1,294
Other assets	104
Accrued expenses and other current liabilities	(32)
Deferred tax liabilities	406
Other liabilities	(31)
(Accumulated deficit) retained earnings	$ 1,459
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)