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, 2016	Apr. 30, 2015	Apr. 30, 2014
Pension Benefits [Member]
Amounts recognized in OCI
Prior service credit (cost)	$ 0	$ 0	$ 0
Net actuarial gain (loss)	(46)	(80)	9
Amortization reclassified to earnings:
Prior service cost (credit)	1	1	1
Net actuarial loss (gain)	27	22	31
Net amount recognized in OCI	(18)	(57)	41
Medical and Life Insurance Benefits [Member]
Amounts recognized in OCI
Prior service credit (cost)	0	16	10
Net actuarial gain (loss)	1	(3)	(3)
Amortization reclassified to earnings:
Prior service cost (credit)	(2)	(2)	0
Net actuarial loss (gain)	1	1	0
Net amount recognized in OCI	$ 0	$ 12	$ 7