Pension and Other Benefit Plans - Pension Plan, Net Periodic Costs and Other Changes (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement/curtailment (gain) loss	$ 519	$ (244)	$ 143
Pension Plans, Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	91	92	88
Interest cost	245	237	253
Expected return on assets	(659)	(651)	(570)
Amortization of transition obligation	0	0	0
Amortization of prior service costs	(8)	(9)	(15)
Contractual termination benefits	13	10	3
Settlement/curtailment (gain) loss	(18)	7	(10)
Recognition of actuarial loss (gain)	537	(252)	153
Net periodic pension expense (income)	$ 201	$ (566)	$ (98)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount or settlement rates	2.40%	2.40%	2.50%
Expected long-term rates of return on assets	5.60%	5.80%	5.30%
Rates of increase in compensation levels	1.70%	2.00%	2.10%
Employer contributions:
2022	$ 155
Benefit Payments:
2022	506
2023	443
2024	444
2025	452
2026	462
2026 and thereafter	2,454
Total	$ 4,761