Pension and Other Benefit Plans - Pension Plan, Net Periodic Costs and Other Changes (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 88	$ 121	$ 23
Interest cost	253	249	82
Expected return on assets	(570)	(534)	(161)
Amortization of transition obligation	0	1	1
Amortization of prior service costs	(15)	(18)	(17)
Contractual termination benefits	3	13	1
Settlement/curtailment gain	(10)	(42)	0
Recognition of actuarial loss (gain)	153	(178)	87
Net periodic pension (income) expense	$ (98)	$ (388)	$ 16
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount or settlement rates	2.50%	2.50%	3.10%
Expected long-term rates of return on assets	5.30%	4.90%	6.30%
Rates of increase in compensation levels	2.10%	2.70%	2.60%
Employer contributions:
2020	$ 82
Benefit Payments:
2020	319
2021	305
2022	358
2023	300
2024	302
2025 and thereafter	1,610
Total	$ 3,194