Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended									12 Months Ended									12 Months Ended						12 Months Ended						12 Months Ended										12 Months Ended							12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2013 Level 1 [Member]	Apr. 30, 2012 Level 1 [Member]	Apr. 30, 2013 Level 2 [Member]	Apr. 30, 2012 Level 2 [Member]	Apr. 30, 2013 Level 3 [Member]	Apr. 30, 2012 Level 3 [Member]	Apr. 30, 2011 Level 3 [Member]	Apr. 30, 2013 Commingled Trust Funds [Member]	Apr. 30, 2012 Commingled Trust Funds [Member]		Apr. 30, 2013 Commingled Trust Funds [Member] Level 2 [Member]	Apr. 30, 2012 Commingled Trust Funds [Member] Level 2 [Member]		Apr. 30, 2013 Commingled Trust Funds [Member] Level 3 [Member]	Apr. 30, 2012 Commingled Trust Funds [Member] Level 3 [Member]		Apr. 30, 2013 Equity Funds [Member]	Apr. 30, 2012 Equity Funds [Member]		Apr. 30, 2013 Equity Funds [Member] Level 2 [Member]	Apr. 30, 2012 Equity Funds [Member] Level 2 [Member]		Apr. 30, 2013 Fixed Income Funds [Member]	Apr. 30, 2012 Fixed Income Funds [Member]		Apr. 30, 2013 Fixed Income Funds [Member] Level 2 [Member]	Apr. 30, 2012 Fixed Income Funds [Member] Level 2 [Member]		Apr. 30, 2013 Real Estate funds [Member]	Apr. 30, 2012 Real Estate funds [Member]		Apr. 30, 2013 Real Estate funds [Member] Level 2 [Member]	Apr. 30, 2012 Real Estate funds [Member] Level 2 [Member]		Apr. 30, 2013 Real Estate funds [Member] Level 3 [Member]	Apr. 30, 2012 Real Estate funds [Member] Level 3 [Member]		Apr. 30, 2011 Real Estate funds [Member] Level 3 [Member]	Apr. 30, 2013 Hedge Funds [Member]	Apr. 30, 2012 Hedge Funds [Member]		Apr. 30, 2013 Hedge Funds [Member] Level 3 [Member]	Apr. 30, 2012 Hedge Funds [Member] Level 3 [Member]		Apr. 30, 2011 Hedge Funds [Member] Level 3 [Member]	Apr. 30, 2013 Private Equity [Member]	Apr. 30, 2012 Private Equity [Member]		Apr. 30, 2013 Private Equity [Member] Level 3 [Member]	Apr. 30, 2012 Private Equity [Member] Level 3 [Member]		Apr. 30, 2011 Private Equity [Member] Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	$ 573	$ 508	$ 0	$ 0	$ 498	$ 441	$ 75	$ 67	$ 49	$ 526	$ 467	[1]	$ 498	$ 441	[1]	$ 28	$ 26	[1]	$ 279	$ 239	[1]	$ 279	$ 239	[1]	$ 199	$ 183	[1]	$ 199	$ 183	[1]	$ 48	$ 45	[1]	$ 20	$ 19	[1]	$ 28	$ 26	[1]	$ 9	$ 26	$ 24	[2]	$ 26	$ 24	[2]	$ 24	$ 21	$ 17	[3]	$ 21	$ 17	[3]	$ 16
Allocation by asset class, Actual (percent)	100.00%	100.00%								92.00%	92.00%	[1]							49.00%	47.00%	[1]				35.00%	36.00%	[1]				8.00%	9.00%	[1]								4.00%	5.00%	[2]					4.00%	3.00%	[3]
Allocation by asset class, Target (percent)	100.00%	100.00%								90.00%	90.00%	[1]							47.00%	47.00%	[1]				35.00%	35.00%	[1]				8.00%	8.00%	[1]								5.00%	5.00%	[2]					5.00%	5.00%	[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.