Leases - Additional Information (Details) $ in Millions	6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019 site	Dec. 31, 2019 USD ($) renewal_option	Mar. 31, 2019 USD ($)	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		113						58
Lease leaseback agreement, term of contract							32 years
Lease leaseback, number of renewal options | renewal_option							0
Operating lease, liability		$ 7,280
Lessor, number of renewal options | renewal_option		0
Residual value of operating leases		$ 3,700						$ 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		$ 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