Pension and Other Benefit Plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Oct. 26, 2018
Defined Benefit Plan Disclosure [Line Items]
Prior service cost				$ 28
Vesting period	1 year
Defined contribution plan contribution		$ 219	$ 245
Shares of company common stock held in defined contribution plan assets (in shares)	3,393,616
Deferred compensation plan, expense	$ 192
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Contractual termination benefits	10	3	13
Recognition of actuarial (gain) loss	(252)	153	(178)
Estimated future amortization of prior service credit	$ 8
Non-employee directors
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation plan, maximum deferral percentage	100.00%
Deferred compensation plan, liability	$ 48	59
Deferred compensation plan, expense	$ 0	$ 2	$ 4