Pension and Other Postretirement Benefits (Change in Fair Value of Level 3 Assets) (Details) - USD ($) $ in Millions		12 Months Ended
		Apr. 30, 2016		Apr. 30, 2015
Change in fair value of Level 3 Assets
Beginning balance		$ 626
Ending balance		594		$ 626
Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance		93		87
Return on assets held at end of year		4		6
Purchases and settlements		24		4
Sales and settlements		(3)		(4)
Ending balance		118		93
Real Estate funds [Member]
Change in fair value of Level 3 Assets
Beginning balance	[1]	56
Ending balance	[1]	59		56
Real Estate funds [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance		36	[1]	32
Return on assets held at end of year		4		4
Purchases and settlements		19		0
Sales and settlements		0		0
Ending balance	[1]	59		36
Hedge Funds [Member]
Change in fair value of Level 3 Assets
Beginning balance	[2]	31
Ending balance	[2]	30		31
Hedge Funds [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance		31	[2]	30
Return on assets held at end of year		(1)		1
Purchases and settlements		0		0
Sales and settlements		0		0
Ending balance	[2]	30		31
Private Equity [Member]
Change in fair value of Level 3 Assets
Beginning balance	[3]	26
Ending balance	[3]	29		26
Private Equity [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance		26	[3]	25
Return on assets held at end of year		1		1
Purchases and settlements		5		4
Sales and settlements		(3)		(4)
Ending balance	[3]	$ 29		$ 26

[1] Commingled trust fund valuations are based on the net asset value (NAV) of the funds as determined by the fund administrators and reviewed by us. NAV represents the underlying assets owned by the fund, minus liabilities and divided by the number of shares or units outstanding.

[2] Hedge fund valuations are based primarily on the NAV of the funds as determined by fund administrators and reviewed by us. During our review, we determine whether it is necessary to adjust a valuation for inherent liquidity and redemption issues that may exist within a fund’s underlying assets or fund unit values.

[3] As of April 30, 2015 and 2016, consists only of limited partnership interests, which are valued at the percentage ownership of total partnership equity as determined by the general partner. These valuations require significant judgment due to the absence of quoted market prices, the inherent lack of liquidity, and the long-term nature of these investments.