Leases - Additional Information (Details) $ in Millions	6 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) site renewal_option	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2008 site	Dec. 31, 2005 site renewal_option	Apr. 01, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Option to extend	20 years
Option to terminate	1 year
Finance lease, right-of-use asset	$ 612						$ 613
Finance lease, accumulated depreciation	95						58
Lease leaseback agreement, term of contract						32 years
Lease leaseback, number of renewal options | renewal_option						0
Operating lease, liability	$ 7,502
Lessor, number of renewal options | renewal_option	0
Residual value of operating leases	$ 3,500						3,200
Rental expense		$ 2,800	$ 2,700	$ 3,100
Cell Sites
Lessee, Lease, Description [Line Items]
Lessee, Lease, Term of Contract	5 years
Lessee, incremental option to extend	5 years
Retail Stores
Lessee, Lease, Description [Line Items]
Lessee, incremental option to extend	5 years
Fleet Vehicles
Lessee, Lease, Description [Line Items]
Lessee, Lease, Term of Contract	3 years
Ground
Lessee, Lease, Description [Line Items]
Operating lease, liability	$ 210						$ 207
Handset
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term	18 months
Tower financing
Lessee, Lease, Description [Line Items]
Sale Leaseback Transaction, Number of Sites | site	1,750				3,000
Lease leaseback arrangement, renewal option duration	20 years
Sale leaseback transaction, net book value		$ 109
Minimum
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term	1 year
Minimum | Cell Sites
Lessee, Lease, Description [Line Items]
Lessee, number of renewal options | renewal_option	1
Minimum | Network Equipment
Lessee, Lease, Description [Line Items]
Lessee, Lease, Term of Contract	5 years
Minimum | Retail Stores
Lessee, Lease, Description [Line Items]
Lessee, Lease, Term of Contract	3 years
Maximum
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term	20 years
Maximum | Cell Sites
Lessee, Lease, Description [Line Items]
Lessee, number of renewal options | renewal_option	4
Maximum | Network Equipment
Lessee, Lease, Description [Line Items]
Option to extend	10 years
Lessee, Lease, Term of Contract	10 years
Maximum | Retail Stores
Lessee, Lease, Description [Line Items]
Lessee, Lease, Term of Contract	10 years
Crown Castle International (CCI)
Lessee, Lease, Description [Line Items]
Sale Leaseback Transaction, Number of Sites | site						5,700
Master Lease Sites
Lessee, Lease, Description [Line Items]
Sale Leaseback Transaction, Number of Sites | site						970