Long-Term Debt, Financing and Capital Lease Obligations (Narrative) (Details)	1 Months Ended					3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2018	Feb. 28, 2017	Oct. 31, 2016 USD ($)	May 31, 2016 USD ($)	Apr. 30, 2016 USD ($)	Dec. 31, 2017	Sep. 30, 2017 USD ($) site years	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) site years	Sep. 30, 2016 USD ($)	Mar. 31, 2017 USD ($)	May 09, 2017	Feb. 02, 2017 USD ($)	Nov. 30, 2015 USD ($)	Dec. 31, 2014 USD ($) tranche
Schedule of Capitalization, Long-term Debt [Line Items]
Maximum Funding Limit															$ 4,300,000,000
Receivables Facility, maximum amount available based on receivables sold to investors, percent							50.00%			50.00%
Receivables Facility, Amount Outstanding							$ 2,400,000,000			$ 2,400,000,000
Receivables Facility, Amount of Funds Available							3,000,000			3,000,000
Receivables Facility, Amount Drawn on Facility										1,600,000,000
Sale Leaseback Transaction, Cumulative Lease Payments										1,100,000,000
Long-term debt issued by 100% owned subsidiary and is fully and unconditionally guaranteed by the parent							22,100,000,000			22,100,000,000
Interest Costs Capitalized										28,000,000	$ 20,000,000
Cash interest payments										1,300,000,000	1,300,000,000
Redeemable notes							27,800,000,000			27,800,000,000
Principal amount of notes with the right to require the Company to repurchase on change of control							$ 21,600,000,000			21,600,000,000
Call premiums paid on debt redemptions										129,000,000	0
Gain (Loss) on Extinguishment of Debt										$ (65,000,000)	0
Long-term Debt, Gross														$ 6,000,000,000
Long-term Debt, Percentage Bearing Variable Interest, Amount														250
Number of cell sites leased back | site							3,000			3,000
Term in years for cell sites leased back renewal options | years							20			20
Payments for (Proceeds from) Businesses and Interest in Affiliates				$ 181,000,000
Ratio Of Total Indebtedness To Adjusted EBITDA							2.5			2.5
cash paid to purchasers under receivables facility											185,000,000
Financing Receivable, Net							$ 1,583,000,000			$ 1,583,000,000		$ 1,764,000,000
Installment Lease Receivables, Net Book Value							3,500,000,000			3,500,000,000
Capital Lease Obligations [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross							323,000,000			323,000,000		3,016,000,000
Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Aggregate principal amount line of credit							3,200,000,000			3,200,000,000
Network Sale-Leaseback [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Sale Leaseback Transaction, Net Book Value					$ 3,000,000,000		2,000,000,000			2,000,000,000
Sale Leaseback Transaction, Net Proceeds, Financing Activities					2,200,000,000
Long-term Debt and Capital Lease Obligations, Repayments of Principal in Next Twelve Months							1,800,000,000			1,800,000,000
Repayments of Debt										117,000,000
Handset Sale-Leaseback Tranche 2 [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Sale Leaseback Transaction, Net Book Value				$ 1,300,000,000			358,000,000			358,000,000
Sale Leaseback Transaction, Net Proceeds, Investing Activities					1,100,000,000
Receivables from sale-leaseback					$ 186,000,000
Repayments of Debt										327,000,000
Handset Sale-Leaseback Tranche 1 [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Sale Leaseback Transaction, Monthly Rental Payments									$ 180,000,000		377,000,000
Tower financing [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Sale Leaseback Transaction, Net Book Value							$ 171,000,000			$ 171,000,000
Minimum [Member] | Capital Lease Obligations [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							2.35%			2.35%
Maximum [Member] | Capital Lease Obligations [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							10.51%			10.51%
Spectrum Assets [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Concentration Risk, Percentage			14.00%
Shentel [Member] | Capital Lease Obligations [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross							$ 153,000,000			$ 153,000,000
Handset Sale-Leaseback Tranche 2 [Member] | MLS 2nd Tranche [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Sale Leaseback Transaction, Net Book Value							58,000,000			58,000,000
Delcredere / Ducroire (D/D) Secured Equipment Credit Facility [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Line of Credit Facility, Increase (Decrease), Net										150,000,000
Delcredere / Ducroire (D/D) Secured Equipment Credit Facility [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Aggregate principal amount line of credit							150,000,000			150,000,000						$ 250,000,000
Repayments of Lines of Credit										3,000,000
Line of Credit Facility, Amount Outstanding							179,000,000			179,000,000
K-sure Secured Equipment Credit Facility [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Aggregate principal amount line of credit																$ 750,000,000
Repayments of Lines of Credit										33,000,000
Number of tranches | tranche																3
Line of Credit Facility, Amount Outstanding							226,000,000			226,000,000
Finnvera Secured Equipment Credit Facility [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Line of Credit Facility, Increase (Decrease), Net										92,000,000
Finnvera Secured Equipment Credit Facility [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Aggregate principal amount line of credit																$ 800,000,000
Repayments of Lines of Credit										85,000,000
Number of tranches | tranche																1
Line of Credit Facility, Amount Outstanding							147,000,000			147,000,000
Export Development Canada Agreement [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Line of Credit Facility, Amount Outstanding												300,000,000
Export Development Canada Agreement [Member] | Tranche three and four [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Line of Credit Facility, Amount Outstanding							300,000,000			300,000,000
Secured Revolving Bank Credit Facility expires in Feb 2021[Member] [Member] | Revolving Credit Facility [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Aggregate principal amount line of credit														$ 2,000,000,000
Line of Credit Facility, Remaining Borrowing Capacity							1,800,000,000			1,800,000,000
Line of Credit Facility, Amount Outstanding							0			0		0
Ratio Of Total Indebtedness To Adjusted EBITDA														6.0
Ratio Of Total Indebtedness To Adjusted EBITDA, Minimum														3.5
Secured Revolving Bank Credit Facility expires in Feb 2021[Member] [Member] | Letter of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Letters of Credit Outstanding, Amount							$ 178,000,000			$ 178,000,000
Sprint Nextel Corporation [Member] | Secured Debt [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							9.25%			9.25%
Long-term Debt, Gross							$ 200,000,000			$ 200,000,000		200,000,000
Sprint Nextel Corporation [Member] | Secured term loan [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross														$ 4,000,000,000
Line of Credit Facility, Amount Outstanding							3,980,000,000			3,980,000,000		4,000,000,000
Debt Instrument, Term		7 years
Sprint Nextel Corporation [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross							4,780,000,000			4,780,000,000		6,080,000,000
Sprint Nextel Corporation [Member] | Guaranteed notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross							$ 2,800,000,000			$ 2,800,000,000		4,000,000,000
Sprint Nextel Corporation [Member] | Minimum [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							6.00%			6.00%
Sprint Nextel Corporation [Member] | Minimum [Member] | Guaranteed notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							7.00%			7.00%
Sprint Nextel Corporation [Member] | Maximum [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							11.50%			11.50%
Sprint Nextel Corporation [Member] | Maximum [Member] | Guaranteed notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							9.00%			9.00%
Spectrum Financing [Member] | Secured Debt [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maximum Borrowing Capacity			$ 7,000,000,000
Debt Instrument, Additional Borrowings Available			$ 3,500,000,000
Sprint Spectrum Co LLC, Sprint Spectrum Co II LLC, Sprint Spectrum Co III LLC [Member] | Secured Debt [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			3.36%				3.36%			3.36%
Long-term Debt, Gross							$ 3,500,000,000			$ 3,500,000,000		3,500,000,000
Debt instrument face amount			$ 3,500,000,000
Debt Instrument, Term			5 years
Secured Debt, Current							875,000,000			875,000,000
Notes due 2017 and Guaranteed Notes Due 2018 [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Call premiums paid on debt redemptions										129,000,000
Debt Instrument, Unamortized Premium							$ 64,000,000			64,000,000
Gain (Loss) on Extinguishment of Debt										$ 65,000,000
Guaranteed Notes Due 2018 [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage													9.00%
Repayments of Debt								$ 1,200,000,000
Notes due 2017 [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							8.375%			8.375%			8.375%
Extinguishment of Debt, Amount							$ 912,000,000
Repayments of Debt								$ 388,000,000
Sprint Corporation [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross							$ 10,500,000,000			$ 10,500,000,000		10,500,000,000
Sprint Corporation [Member] | Minimum [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							7.13%			7.13%
Sprint Corporation [Member] | Maximum [Member] | Senior Notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							7.88%			7.88%
Clearwire Communications LLC [Member] | Exchangeable notes [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Redemption price (as a percent)										100.00%
Debt Instrument, Interest Rate, Stated Percentage							8.25%			8.25%
Long-term Debt, Gross							$ 629,000,000			$ 629,000,000		629,000,000
Financing Receivable
Schedule of Capitalization, Long-term Debt [Line Items]
Cash received for sale of receivables under receivables facility											40,000,000
Cash Flows Between Transferor and Transferee, Proceeds from Collections Reinvested in Revolving Period Transfers											$ 255,000,000
Accounts and notes receivable, net
Schedule of Capitalization, Long-term Debt [Line Items]
Financing Receivable, Net							1,269,000,000			1,269,000,000		1,195,000,000
Accounts and notes receivable, net | Wireless Services and Installment Receivables [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Financing Receivable, Net							3,000,000,000			3,000,000,000
Other assets
Schedule of Capitalization, Long-term Debt [Line Items]
Financing Receivable, Net							314,000,000			314,000,000		$ 569,000,000
Interest Rate Swap [Member] | Sprint Nextel Corporation [Member] | Secured term loan [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Derivative, Notional Amount														$ 2,000,000,000
Derivative Liability, Fair Value, Gross Liability							$ 9,000,000			$ 9,000,000
Derivative, Term of Contract												5 years
Scenario, Forecast [Member] | Wireless Service [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Purchase Commitment Allocation	26.00%					26.00%
Scenario, Forecast [Member] | Installment Receivables [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Purchase Commitment Allocation	18.00%					28.00%
Scenario, Forecast [Member] | Future lease receivables [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Purchase Commitment Allocation	56.00%					46.00%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Basis Spread on Variable Rate										1.25%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Basis Spread on Variable Rate										4.25%
Base Rate [Member] | Minimum [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Basis Spread on Variable Rate										0.25%
Base Rate [Member] | Maximum [Member] | Line of Credit [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Basis Spread on Variable Rate										3.25%