Pension and Other Benefit Plans - Pension Plan, Net Periodic Costs and Other Changes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Contractual termination benefit	$ 6		$ 17
Pension Plans, Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	23	$ 22	46	$ 45
Interest cost	57	63	117	128
Expected return on assets	(154)	(138)	(315)	(287)
Amortization of prior service costs	(2)	(6)	(4)	(7)
Contractual termination benefit	6	0	17	0
Curtailment gain	0	0	0	(1)
Net periodic pension income	$ (70)	$ (59)	$ (139)	$ (122)
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%	2.10%