Guarantor Financial Information (CONDENSED CONSOLIDATING BALANCE SHEET) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Apr. 01, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	$ 1,021	$ 997	$ 2,044	$ 1,974
Current Assets
Cash and Cash Equivalents	5,726	4,871	5,726	4,871			$ 6,610		$ 2,942
Short-term Investments	3,186		3,186				2,354
Accounts and notes receivable, net	3,555		3,555			$ 3,808	3,711
Due from Affiliate, Current	0		0				0
Device and accessory inventory	859		859			979	1,003
Prepaid expenses and other current assets	1,121		1,121			846	575
Total current assets	14,447		14,447				14,253
Investments in Subsidiaries	0		0				0
Property, plant and equipment, net	20,816		20,816				19,925
Costs to acquire a customer contract	1,379		1,379			1,219
Due from Consolidated Affiliate	0		0				0
Due from Affiliate, Noncurrent	0		0				0
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	6,598		6,598				6,586
FCC licenses and other	41,373		41,373				41,309
Definite-lived intangible assets, net	2,075		2,075				2,465
Other assets	1,163		1,163			964	921
Total assets	87,851		87,851				85,459
Current Liabilities
Accounts Payable, Current	4,210		4,210				3,409
Accrued expenses and other current liabilities	3,370		3,370			3,927	3,962
Current portion of long-term debt, financing and capital lease obligations	5,346		5,346				3,429
Due to Affiliate, Current	0		0				0
Total current liabilities	12,926		12,926				10,800
Long-term debt, financing and capital lease obligations	35,329		35,329				37,463
Note due to consolidated affiliate	0		0				0
Deferred tax liabilities	7,704		7,704			7,660	7,294
Other liabilities	3,428		3,428			$ 3,451	3,483
Due to affiliate	0		0				0
Total liabilities	59,387		59,387				59,040
Commitments and contingencies
Total stockholders' equity	28,401		28,401				26,356
Stockholders' Equity Attributable to Noncontrolling Interest	63		63				63
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	28,464	19,020	28,464	19,020	$ 27,945		26,419	$ 19,015	18,808
Liabilities and Equity	87,851		87,851				85,459
Depreciation, equipment rentals	1,181	888	2,317	1,742
Consolidation, Eliminations [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	0	0	0	0
Current Assets
Cash and Cash Equivalents	0	0	0	0			0		0
Short-term Investments	0		0				0
Accounts and notes receivable, net	(329)		(329)				(347)
Due from Affiliate, Current	(424)		(424)				(424)
Device and accessory inventory	0		0				0
Prepaid expenses and other current assets	0		0				0
Total current assets	(753)		(753)				(771)
Investments in Subsidiaries	(47,413)		(47,413)				(45,136)
Property, plant and equipment, net	0		0				0
Costs to acquire a customer contract	0		0
Due from Consolidated Affiliate	(1,298)		(1,298)				(595)
Due from Affiliate, Noncurrent	(35,438)		(35,438)				(35,878)
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	0		0				0
FCC licenses and other	0		0				0
Definite-lived intangible assets, net	0		0				0
Other assets	0		0				0
Total assets	(84,902)		(84,902)				(82,380)
Current Liabilities
Accounts Payable, Current	0		0				0
Accrued expenses and other current liabilities	(329)		(329)				(347)
Current portion of long-term debt, financing and capital lease obligations	0		0				0
Due to Affiliate, Current	(424)		(424)				(424)
Total current liabilities	(753)		(753)				(771)
Long-term debt, financing and capital lease obligations	0		0				0
Note due to consolidated affiliate	(35,438)		(35,438)				(35,878)
Deferred tax liabilities	0		0				0
Other liabilities	0		0				0
Due to affiliate	(1,298)		(1,298)				(595)
Total liabilities	(37,489)		(37,489)				(37,244)
Commitments and contingencies
Total stockholders' equity	(47,413)		(47,413)				(45,136)
Stockholders' Equity Attributable to Noncontrolling Interest	0		0				0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(47,413)		(47,413)				(45,136)
Liabilities and Equity	(84,902)		(84,902)				(82,380)
Depreciation, equipment rentals	0	0	0	0
Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	0	0	0	0
Current Assets
Cash and Cash Equivalents	0	0	0	0			0		0
Short-term Investments	0		0				0
Accounts and notes receivable, net	96		96				99
Due from Affiliate, Current	0		0				0
Device and accessory inventory	0		0				0
Prepaid expenses and other current assets	1		1				5
Total current assets	97		97				104
Investments in Subsidiaries	28,110		28,110				26,351
Property, plant and equipment, net	0		0				0
Costs to acquire a customer contract	0		0
Due from Consolidated Affiliate	291		291				1
Due from Affiliate, Noncurrent	11,871		11,871				11,887
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	0		0				0
FCC licenses and other	0		0				0
Definite-lived intangible assets, net	0		0				0
Other assets	0		0				0
Total assets	40,369		40,369				38,343
Current Liabilities
Accounts Payable, Current	0		0				0
Accrued expenses and other current liabilities	97		97				100
Current portion of long-term debt, financing and capital lease obligations	0		0				0
Due to Affiliate, Current	0		0				0
Total current liabilities	97		97				100
Long-term debt, financing and capital lease obligations	11,871		11,871				11,887
Note due to consolidated affiliate	0		0				0
Deferred tax liabilities	0		0				0
Other liabilities	0		0				0
Due to affiliate	0		0				0
Total liabilities	11,968		11,968				11,987
Commitments and contingencies
Total stockholders' equity	28,401		28,401				26,356
Stockholders' Equity Attributable to Noncontrolling Interest	0		0				0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	28,401		28,401				26,356
Liabilities and Equity	40,369		40,369				38,343
Depreciation, equipment rentals	0	0	0	0
Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	0	0	0	0
Current Assets
Cash and Cash Equivalents	5,256	4,319	5,256	4,319			6,222		2,461
Short-term Investments	3,186		3,186				2,354
Accounts and notes receivable, net	233		233				248
Due from Affiliate, Current	424		424				424
Device and accessory inventory	0		0				0
Prepaid expenses and other current assets	8		8				9
Total current assets	9,107		9,107				9,257
Investments in Subsidiaries	19,303		19,303				18,785
Property, plant and equipment, net	0		0				0
Costs to acquire a customer contract	0		0
Due from Consolidated Affiliate	1,007		1,007				0
Due from Affiliate, Noncurrent	23,567		23,567				23,991
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	0		0				0
FCC licenses and other	0		0				0
Definite-lived intangible assets, net	0		0				0
Other assets	206		206				185
Total assets	53,190		53,190				52,218
Current Liabilities
Accounts Payable, Current	0		0				0
Accrued expenses and other current liabilities	314		314				341
Current portion of long-term debt, financing and capital lease obligations	2,021		2,021				1,832
Due to Affiliate, Current	0		0				0
Total current liabilities	2,335		2,335				2,173
Long-term debt, financing and capital lease obligations	10,104		10,104				10,381
Note due to consolidated affiliate	11,871		11,871				11,887
Deferred tax liabilities	0		0				0
Other liabilities	770		770				831
Due to affiliate	0		0				595
Total liabilities	25,080		25,080				25,867
Commitments and contingencies
Total stockholders' equity	28,110		28,110				26,351
Stockholders' Equity Attributable to Noncontrolling Interest	0		0				0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	28,110		28,110				26,351
Liabilities and Equity	53,190		53,190				52,218
Depreciation, equipment rentals	0	0	0	0
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	1,021	997	2,044	1,974
Other Additional Capital	1,300	231	1,300	231
Current Assets
Cash and Cash Equivalents	470	552	470	552			388		$ 481
Short-term Investments	0		0				0
Accounts and notes receivable, net	3,555		3,555				3,711
Due from Affiliate, Current	0		0				0
Device and accessory inventory	859		859				1,003
Prepaid expenses and other current assets	1,112		1,112				561
Total current assets	5,996		5,996				5,663
Investments in Subsidiaries	0		0				0
Property, plant and equipment, net	20,816		20,816				19,925
Costs to acquire a customer contract	1,379		1,379
Due from Consolidated Affiliate	0		0				594
Due from Affiliate, Noncurrent	0		0				0
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	6,598		6,598				6,586
FCC licenses and other	41,373		41,373				41,309
Definite-lived intangible assets, net	2,075		2,075				2,465
Other assets	957		957				736
Total assets	79,194		79,194				77,278
Current Liabilities
Accounts Payable, Current	4,210		4,210				3,409
Accrued expenses and other current liabilities	3,288		3,288				3,868
Current portion of long-term debt, financing and capital lease obligations	3,325		3,325				1,597
Due to Affiliate, Current	424		424				424
Total current liabilities	11,247		11,247				9,298
Long-term debt, financing and capital lease obligations	13,354		13,354				15,195
Note due to consolidated affiliate	23,567		23,567				23,991
Deferred tax liabilities	7,704		7,704				7,294
Other liabilities	2,658		2,658				2,652
Due to affiliate	1,298		1,298				0
Total liabilities	59,828		59,828				58,430
Commitments and contingencies
Total stockholders' equity	19,303		19,303				18,785
Stockholders' Equity Attributable to Noncontrolling Interest	63		63				63
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	19,366		19,366				18,848
Liabilities and Equity	79,194		79,194				$ 77,278
Depreciation, equipment rentals	$ 1,181	$ 888	$ 2,317	$ 1,742