Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) - USD ($) $ in Millions		Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		$ 623	$ 594
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		32	29	$ 26
Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	206	197
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	229	197
Real Estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	63	59
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	7	4
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[2]	8	30
Included in Fair Value Hierarchy [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		110	107
Included in Fair Value Hierarchy [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		78	78
Included in Fair Value Hierarchy [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Included in Fair Value Hierarchy [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		32	29
Included in Fair Value Hierarchy [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	32	29
Included in Fair Value Hierarchy [Member] | Private Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	0	0
Included in Fair Value Hierarchy [Member] | Private Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	0	0
Included in Fair Value Hierarchy [Member] | Private Equity [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	32	29
Included in Fair Value Hierarchy [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		78	78
Included in Fair Value Hierarchy [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		78	78
Included in Fair Value Hierarchy [Member] | Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Included in Fair Value Hierarchy [Member] | 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] Limited partnership interests 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.