Guarantor Financial Information (CONDENSED CONSOLIDATING BALANCE SHEET) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Jun. 30, 2018	Apr. 01, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	$ 1,088	$ 987	$ 3,132	$ 2,961
Current Assets
Cash and Cash Equivalents	6,191	4,495	6,191	4,495				$ 6,610			$ 2,942
Short-term Investments	632		632					2,354
Accounts and notes receivable, net	3,455		3,455				$ 3,808	3,711
Due from Affiliate, Current	0		0					0
Device and accessory inventory	919		919				979	1,003
Prepaid expenses and other current assets	1,199		1,199				846	575
Total current assets	12,396		12,396					14,253
Investments in Subsidiaries	0		0					0
Property, plant and equipment, net	21,422		21,422					19,925
Costs to acquire a customer contract	1,497		1,497				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	41,448		41,448					41,309
Definite-lived intangible assets, net	1,915		1,915					2,465
Other assets	1,128		1,128				964	921
Total assets	86,404		86,404					85,459
Current Liabilities
Accounts Payable, Current	3,637		3,637					3,409
Accrued expenses and other current liabilities	3,467		3,467				3,927	3,962
Current portion of long-term debt, financing and capital lease obligations	3,596		3,596					3,429
Due to Affiliate, Current	0		0					0
Total current liabilities	10,700		10,700					10,800
Long-term debt, financing and capital lease obligations	36,288		36,288					37,463
Note due to consolidated affiliate	0		0					0
Deferred tax liabilities	7,684		7,684				7,660	7,294
Other liabilities	3,403		3,403				$ 3,451	3,483
Due to affiliate	0		0					0
Total liabilities	58,075		58,075					59,040
Commitments and contingencies
Total stockholders' equity	28,270		28,270					26,356
Stockholders' Equity Attributable to Noncontrolling Interest	59		59					63
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	28,329	26,305	28,329	26,305	$ 28,464	$ 27,945		26,419	$ 19,020	$ 19,015	18,808
Liabilities and Equity	86,404		86,404					85,459
Depreciation, equipment rentals	1,137	990	3,454	2,732
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	(710)		(710)					(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	(1,134)		(1,134)					(771)
Investments in Subsidiaries	(47,297)		(47,297)					(45,136)
Property, plant and equipment, net	0		0					0
Costs to acquire a customer contract	0		0
Due from Consolidated Affiliate	(2,138)		(2,138)					(595)
Due from Affiliate, Noncurrent	(35,444)		(35,444)					(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	(86,013)		(86,013)					(82,380)
Current Liabilities
Accounts Payable, Current	0		0					0
Accrued expenses and other current liabilities	(710)		(710)					(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	(1,134)		(1,134)					(771)
Long-term debt, financing and capital lease obligations	0		0					0
Note due to consolidated affiliate	(35,444)		(35,444)					(35,878)
Deferred tax liabilities	0		0					0
Other liabilities	0		0					0
Due to affiliate	(2,138)		(2,138)					(595)
Total liabilities	(38,716)		(38,716)					(37,244)
Commitments and contingencies
Total stockholders' equity	(47,297)		(47,297)					(45,136)
Stockholders' Equity Attributable to Noncontrolling Interest	0		0					0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(47,297)		(47,297)					(45,136)
Liabilities and Equity	(86,013)		(86,013)					(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	233		233					99
Due from Affiliate, Current	0		0					0
Device and accessory inventory	0		0					0
Prepaid expenses and other current assets	0		0					5
Total current assets	233		233					104
Investments in Subsidiaries	27,983		27,983					26,351
Property, plant and equipment, net	0		0					0
Costs to acquire a customer contract	0		0
Due from Consolidated Affiliate	289		289					1
Due from Affiliate, Noncurrent	11,877		11,877					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	40,382		40,382					38,343
Current Liabilities
Accounts Payable, Current	0		0					0
Accrued expenses and other current liabilities	235		235					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	235		235					100
Long-term debt, financing and capital lease obligations	11,877		11,877					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	12,112		12,112					11,987
Commitments and contingencies
Total stockholders' equity	28,270		28,270					26,356
Stockholders' Equity Attributable to Noncontrolling Interest	0		0					0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	28,270		28,270					26,356
Liabilities and Equity	40,382		40,382					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	5,800	3,948	5,800	3,948				6,222			2,461
Short-term Investments	632		632					2,354
Accounts and notes receivable, net	477		477					248
Due from Affiliate, Current	424		424					424
Device and accessory inventory	0		0					0
Prepaid expenses and other current assets	15		15					9
Total current assets	7,348		7,348					9,257
Investments in Subsidiaries	19,314		19,314					18,785
Property, plant and equipment, net	0		0					0
Costs to acquire a customer contract	0		0
Due from Consolidated Affiliate	1,849		1,849					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	70		70					185
Total assets	52,148		52,148					52,218
Current Liabilities
Accounts Payable, Current	0		0					0
Accrued expenses and other current liabilities	349		349					341
Current portion of long-term debt, financing and capital lease obligations	351		351					1,832
Due to Affiliate, Current	0		0					0
Total current liabilities	700		700					2,173
Long-term debt, financing and capital lease obligations	10,837		10,837					10,381
Note due to consolidated affiliate	11,877		11,877					11,887
Deferred tax liabilities	0		0					0
Other liabilities	751		751					831
Due to affiliate	0		0					595
Total liabilities	24,165		24,165					25,867
Commitments and contingencies
Total stockholders' equity	27,983		27,983					26,351
Stockholders' Equity Attributable to Noncontrolling Interest	0		0					0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	27,983		27,983					26,351
Liabilities and Equity	52,148		52,148					52,218
Depreciation, equipment rentals	0	0	0	0
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Depreciation, network and other	1,088	987	3,132	2,961
Other Additional Capital	1,100	4,700	1,100	4,700
Current Assets
Cash and Cash Equivalents	391	547	391	547				388			$ 481
Short-term Investments	0		0					0
Accounts and notes receivable, net	3,455		3,455					3,711
Due from Affiliate, Current	0		0					0
Device and accessory inventory	919		919					1,003
Prepaid expenses and other current assets	1,184		1,184					561
Total current assets	5,949		5,949					5,663
Investments in Subsidiaries	0		0					0
Property, plant and equipment, net	21,422		21,422					19,925
Costs to acquire a customer contract	1,497		1,497
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	41,448		41,448					41,309
Definite-lived intangible assets, net	1,915		1,915					2,465
Other assets	1,058		1,058					736
Total assets	79,887		79,887					77,278
Current Liabilities
Accounts Payable, Current	3,637		3,637					3,409
Accrued expenses and other current liabilities	3,593		3,593					3,868
Current portion of long-term debt, financing and capital lease obligations	3,245		3,245					1,597
Due to Affiliate, Current	424		424					424
Total current liabilities	10,899		10,899					9,298
Long-term debt, financing and capital lease obligations	13,574		13,574					15,195
Note due to consolidated affiliate	23,567		23,567					23,991
Deferred tax liabilities	7,684		7,684					7,294
Other liabilities	2,652		2,652					2,652
Due to affiliate	2,138		2,138					0
Total liabilities	60,514		60,514					58,430
Commitments and contingencies
Total stockholders' equity	19,314		19,314					18,785
Stockholders' Equity Attributable to Noncontrolling Interest	59		59					63
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	19,373		19,373					18,848
Liabilities and Equity	79,887		79,887					$ 77,278
Depreciation, equipment rentals	$ 1,137	$ 990	$ 3,454	$ 2,732