Pension and Other Benefit Plans - Pension Plan, Net Periodic Costs and Other Changes (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 23	$ 32
Interest cost	65	60
Expected return on assets	(149)	(128)
Amortization of prior service costs	(1)	(4)
Contractual termination benefit	0	9
Curtailment gain	(1)	0
Net periodic pension income	$ (63)	$ (31)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount or settlement rates	2.30%	2.50%
Expected long-term rates of return on assets	5.30%	4.90%
Rates of increase in compensation levels	2.10%	2.70%