Pension and Other Postretirement Benefits (Changes in Funded Status of Benefit Plans Recognized in Other Comprehensive (Income) Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Pension Benefits [Member]
Amounts recognized in OCI
Prior service credit (cost)	$ (1)	$ 0	$ 0
Net actuarial gain (loss)	24	(46)	(80)
Amortization reclassified to earnings:
Prior service cost (credit)	1	1	1
Net actuarial loss (gain)	26	27	22
Net amount recognized in OCI	50	(18)	(57)
Medical and Life Insurance Benefits [Member]
Amounts recognized in OCI
Prior service credit (cost)	4	0	16
Net actuarial gain (loss)	0	1	(3)
Amortization reclassified to earnings:
Prior service cost (credit)	(3)	(2)	(2)
Net actuarial loss (gain)	1	1	1
Net amount recognized in OCI	$ 2	$ 0	$ 12