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, 2013		Apr. 30, 2012
Change in fair value of Level 3 Assets
Ending balance	$ 573		$ 508
Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	67		49
Return on assets held at end of year	6		2
Return on assets sold during year			1
Purchases and settlements	4		17
Sales and settlements	(2)		(2)
Ending balance	75		67
Real Estate funds [Member]
Change in fair value of Level 3 Assets
Ending balance	48		45	[1]
Real Estate funds [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	26	[1]	9
Return on assets held at end of year	2		3
Return on assets sold during year			0
Purchases and settlements	0		14
Sales and settlements	0		0
Ending balance	28		26	[1]
Hedge Funds [Member]
Change in fair value of Level 3 Assets
Ending balance	26		24	[2]
Hedge Funds [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	24	[2]	24
Return on assets held at end of year	2		(1)
Return on assets sold during year			1
Purchases and settlements	0		0
Sales and settlements	0		0
Ending balance	26		24	[2]
Private Equity [Member]
Change in fair value of Level 3 Assets
Ending balance	21		17	[3]
Private Equity [Member] | Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	17	[3]	16
Return on assets held at end of year	2		0
Return on assets sold during year			0
Purchases and settlements	4		3
Sales and settlements	(2)		(2)
Ending balance	$ 21		$ 17	[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, 2012 and 2013, 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.