Severance and Exit Costs (Details) - USD ($) $ in Millions		12 Months Ended
		Mar. 31, 2017		Mar. 31, 2016		Mar. 31, 2015
Restructuring Cost and Reserve [Line Items]
Severance costs		$ 20	[1]	$ 216	[2]
Restructuring charges		68		391
Lease [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		17	[3]	156	[4]
Access [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		31	[5]	19	[6]
Wireless [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs		19		191
Wireless [Member] | Lease [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	[3]	14
Wireline [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs		1		25
Wireline [Member] | Lease [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	[3]	3
Other Operating Income (Expense) [Member] | Wireless [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, accrual adjustment				20		$ 41
cost of products [Member] | Wireless [Member] | Access [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges				17
cost of products [Member] | Wireline [Member] | Access [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		2	[5]	$ 2
Severance and exit costs [Member] | Access [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	[5]	29
Severance and exit costs [Member] | Wireless [Member] | Access [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	[5]	12
Severance and exit costs [Member] | Wireline [Member] | Access [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	[5]	$ 17

[1] (2)For the year ended March 31, 2017, we recognized costs of $20 million ($19 million Wireless, $1 million Wireline).

[2] (5)For the year ended March 31, 2016, we recognized costs of $216 million ($191 million Wireless, $25 million Wireline).

[3] (1)For the year ended March 31, 2017, we recognized costs of $17 million ($14 million Wireless, $3 million Wireline).

[4] (4)We recognized costs of $176 million (Wireless only) for the year ended March 31, 2016, which were offset by $20 million of income (Wireless only) resulting from a revision to our estimate of a previously recorded reserve.

[5] (3)For the year ended March 31, 2017, we recognized $2 million (solely attributable to Wireline) as "Cost of services" and $29 million ($12 million Wireless, $17 million Wireline) as "Severance and exit costs."

[6] (6)For the year ended March 31, 2016, we recognized $2 million (solely attributable to Wireline) as "Cost of services" and $17 million (solely attributable to Wireless) as "Severance and exit costs".