Postretirement benefits (Details) (USD $) In Millions, unless otherwise specified	2 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended		12 Months Ended	3 Months Ended
	Dec. 31, 2010	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2012 Pension Benefit [Member]	Apr. 30, 2011 Pension Benefit [Member]	Apr. 30, 2012 Pension Benefit [Member]	Apr. 30, 2011 Pension Benefit [Member]	Apr. 30, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member]	Apr. 30, 2012 Early Retiree Reinsurance Program [Member]	Apr. 30, 2012 Early Retiree Reinsurance Program [Member]	Apr. 30, 2011 Early Retiree Reinsurance Program [Member]	Oct. 31, 1993 Retiree Supplemental Benefit Trust [Member]	Jan. 31, 2011 Location Fort Wayne [Member] Pension and Other Postretirement Contractual Termination Benefits [Member] Truck [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefits Paid						$ 53	$ 31	$ 82	$ 52
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year						108		108
Defined Benefit Plan, Contributions by Plan Participants											0	3	5
Restructuring charges (benefit)		20	2	20	24										5
Defined Contribution Plan, Cost Recognized	10	14	8	24	18
Contribution To The Trust Shares														25.5
Profit Sharing Accruals														0
Defined Benefit Plan, Contributions by Employer										1
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year										$ 18