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, 2020	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 92	$ 88	$ 121
Interest cost	237	253	249
Expected return on assets	(651)	(570)	(534)
Amortization of transition obligation	0	0	1
Amortization of prior service costs	(9)	(15)	(18)
Contractual termination benefits	10	3	13
Settlement/curtailment gain	7	(10)	(42)
Recognition of actuarial (gain) loss	(252)	153	(178)
Net periodic pension (income) expense	$ (566)	$ (98)	$ (388)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount or settlement rates	2.40%	2.50%	2.50%
Expected long-term rates of return on assets	5.80%	5.30%	4.90%
Rates of increase in compensation levels	2.00%	2.10%	2.70%
Employer contributions:
2021	$ 74
Benefit Payments:
2021	393
2022	447
2023	392
2024	394
2025	404
2025 and thereafter	2,112
Total	$ 4,142