Pension and Other Postretirement Benefits (Change in Fair Value of Level 3 Assets) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Apr. 30, 2015		Apr. 30, 2014
Change in fair value of Level 3 Assets
Ending balance	$ 626		$ 605
Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	87		75
Return on assets held at end of year	6		10
Purchases and settlements	4		5
Sales and settlements	(4)		(3)
Ending balance	93		87
Real Estate funds [Member]
Change in fair value of Level 3 Assets
Ending balance	56	[1]	53	[1]
Real Estate funds [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	32	[1]	28
Return on assets held at end of year	4		4
Purchases and settlements	0		0
Sales and settlements	0		0
Ending balance	36	[1]	32	[1]
Hedge Funds [Member]
Change in fair value of Level 3 Assets
Ending balance	31	[2]	30	[2]
Hedge Funds [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	30	[2]	26
Return on assets held at end of year	1		2
Purchases and settlements	0		2
Sales and settlements	0		0
Ending balance	31	[2]	30	[2]
Private Equity [Member]
Change in fair value of Level 3 Assets
Ending balance	26	[3]	25	[3]
Private Equity [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	25	[3]	21
Return on assets held at end of year	1		4
Purchases and settlements	4		3
Sales and settlements	(4)		(3)
Ending balance	$ 26	[3]	$ 25	[3]

[1] Commingled trust fund valuations are based on the net asset value (NAV) of the funds as determined by the administrator of the fund 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, 2014 and 2015, 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.