Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) - USD ($) $ in Millions		Apr. 30, 2016		Apr. 30, 2015		Apr. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		$ 594		$ 626
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		78		76
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		398		457
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		118		93		$ 87
Commingled Trust Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	457		493
Commingled Trust Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Commingled Trust Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	398		457
Commingled Trust Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	59		36
Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	197		248
Equity Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Equity Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	197		248
Equity Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	197		185
Fixed Income Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Fixed Income Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	197		185
Fixed Income Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Real Estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	59		56
Real Estate funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Real Estate funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		20
Real Estate funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		59	[1]	36	[1]	32
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	4		4
Short-term Investments [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Short-term Investments [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	4		4
Short-term Investments [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0		0
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[2]	30		31
Hedge Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[2]	0		0
Hedge Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[2]	0		0
Hedge Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		30	[2]	31	[2]	30
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	29		26
Private Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	0		0
Private Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	0		0
Private Equity [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		29	[3]	26	[3]	$ 25
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		78		76
Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		78		76
Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0		0
Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		$ 0		$ 0

[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.