Pension and Other Benefit Plans - Pension Plan, Net Periodic Costs and Other Changes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Contractual termination benefit	$ 0		$ 17
Pension Plans, Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	23	$ 21	69	$ 66
Interest cost	60	62	177	190
Expected return on assets	(161)	(139)	(476)	(426)
Amortization of prior service costs	(2)	(4)	(6)	(11)
Contractual termination benefit	0	2	17	2
Curtailment gain	0	0	0	(1)
Net periodic pension income	$ (80)	$ (58)	$ (219)	$ (180)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount or settlement rates			2.40%	2.30%
Expected long-term rates of return on assets			5.80%	5.30%
Rates of increase in compensation levels			2.00%	2.00%