Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Apr. 30, 2015		Apr. 30, 2014		Apr. 30, 2013
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	$ 626		$ 605
Allocation by asset class, Actual (percent)	100.00%		100.00%
Allocation by asset class, Target (percent)	100.00%		100.00%
Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	76		63
Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	457		455
Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	93		87		75
Commingled Trust Funds [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	493	[1]	487	[1]
Allocation by asset class, Actual (percent)	79.00%	[1]	80.00%	[1]
Allocation by asset class, Target (percent)	81.00%	[1]	81.00%	[1]
Commingled Trust Funds [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]	0	[1]
Commingled Trust Funds [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	457	[1]	455	[1]
Commingled Trust Funds [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	36	[1]	32	[1]
Equity Funds [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	248	[1]	235	[1]
Allocation by asset class, Actual (percent)	39.00%	[1]	38.00%	[1]
Allocation by asset class, Target (percent)	38.00%	[1]	38.00%	[1]
Equity Funds [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]	0	[1]
Equity Funds [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	248	[1]	235	[1]
Equity Funds [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]	0	[1]
Fixed Income Funds [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	185	[1]	196	[1]
Allocation by asset class, Actual (percent)	30.00%	[1]	32.00%	[1]
Allocation by asset class, Target (percent)	35.00%	[1]	35.00%	[1]
Fixed Income Funds [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]	0	[1]
Fixed Income Funds [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	185	[1]	196	[1]
Fixed Income Funds [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]	0	[1]
Real Estate funds [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	56	[1]	53	[1]
Allocation by asset class, Actual (percent)	9.00%	[1]	9.00%	[1]
Allocation by asset class, Target (percent)	8.00%	[1]	8.00%	[1]
Real Estate funds [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]	0	[1]
Real Estate funds [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	20	[1]	21	[1]
Real Estate funds [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	36	[1]	32	[1]	28
Short-term Investments [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	4	[1]
Allocation by asset class, Actual (percent)	1.00%	[1]
Allocation by asset class, Target (percent)	0.00%	[1]
Short-term Investments [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]
Short-term Investments [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	4	[1]
Short-term Investments [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[1]
Hedge Funds [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	31	[2]	30	[2]
Allocation by asset class, Actual (percent)	5.00%	[2]	5.00%	[2]
Allocation by asset class, Target (percent)	5.00%	[2]	5.00%	[2]
Hedge Funds [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[2]	0	[2]
Hedge Funds [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[2]	0	[2]
Hedge Funds [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	31	[2]	30	[2]	26
Private Equity [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	26	[3]	25	[3]
Allocation by asset class, Actual (percent)	4.00%	[3]	4.00%	[3]
Allocation by asset class, Target (percent)	5.00%	[3]	5.00%	[3]
Private Equity [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[3]	0	[3]
Private Equity [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0	[3]	0	[3]
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	26	[3]	25	[3]	21
Equity Securities [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	76
Allocation by asset class, Actual (percent)	12.00%
Allocation by asset class, Target (percent)	9.00%
Equity Securities [Member] | Level 1 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	76
Equity Securities [Member] | Level 2 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	0
Equity Securities [Member] | Level 3 [Member]
Fair value of pension plan assets by category, as well as the actual and target allocations
Total, Plan Assets	$ 0

[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.