Transactions Involving Related Parties - Summary of Income Statement Impact of Transactions with Parent and Affiliated Companies (Detail) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Subsidy income	$ 277	$ 266	$ 821	$ 801
Interest expense:
Related party debt	3	7	12	20
Other income:
VSC administration fees	24	24	73	72
General and administrative expenses:
Support Compensation Agreement fees	5	4	14	13
Benefit plan expenses	3	2	8	7
Shared services	$ 15	$ 19	$ 46	$ 45