Summary of Significant Accounting Policies and Other Information New Accounting Pronouncements - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2019	Apr. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capitalized Contract Cost, Net	$ 1,559	$ 1,219
Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption, Before Tax		1,700
Cumulative Effect of New Accounting Principle in Period of Adoption		1,300
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption, Before Tax		0
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Wireless Service [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	24 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Leasing contracts [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	30 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Leasing contracts [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	18 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Handsets [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	30 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Handsets [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	24 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Accessories [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	18 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Accessories [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Wireline [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	36 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Wireline [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	24 months