Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) - Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	$ 13,837	$ 2,166	$ 2,032
Balance	11,725	13,837	2,166
Foreign currency translation adjustments
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	(261)	(458)	(399)
Current-period other comprehensive (loss) income	(256)	197	(59)
Amounts reclassified from accumulated other comprehensive (loss) income, net of taxes	0	0	0
Balance	(517)	(261)	(458)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	9
Current-period other comprehensive (loss) income	(22)
Amounts reclassified from accumulated other comprehensive (loss) income, net of taxes	10
Balance	(3)	9
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance		20	(1)
Current-period other comprehensive (loss) income		(11)	21
Amounts reclassified from accumulated other comprehensive (loss) income, net of taxes		0	0
Balance			20
Available-for-sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	9	0	0
Current-period other comprehensive (loss) income	0	9	0
Amounts reclassified from accumulated other comprehensive (loss) income, net of taxes	0	0	0
Balance	9	9	0
Pension and other post-retirement benefit plans:
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	301	276	289
Current-period other comprehensive (loss) income	(21)	0	(2)
Amounts reclassified from accumulated other comprehensive (loss) income, net of taxes	(13)	25	(11)
Balance	267	301	276
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]
Balance	58	(162)	(111)
Current-period other comprehensive (loss) income	(299)	195	(40)
Amounts reclassified from accumulated other comprehensive (loss) income, net of taxes	(3)	25	(11)
Balance	$ (244)	$ 58	$ (162)