Pensions and Other Postretirement Benefits (Details 5) (USD $) In Thousands	3 Months Ended	12 Months Ended
Jul. 31, 2011	Jul. 31, 2010	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009
Company's Canadian pension and other postretirement benefit plans
Company contributions	$ (3,691)	$ (10,544)	$ (16,779)	$ (4,436)	$ (34,665)
Defined Benefit Pension Plans [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of the year			503,346	450,728	362,720
Fair value of plan assets at end of the year			407,600	367,322	300,482
Funded status of the plans			(95,746)	(83,406)
Service cost	2,062	1,858	7,504	5,755	5,871
Interest cost	6,523	6,346	25,491	24,788	26,263
Expected return on plan assets	6,885	6,657	26,848	22,894	29,905
Curtailment	0	3,910	4,095	0	0
Termination benefit cost	0	7,462	8,395	0	0
Company contributions			16,779	4,436
Participant contributions			498	410
Benefits paid			(30,502)	(25,296)
Actual return on plan assets			45,743	73,604
Net periodic benefit cost	4,201	14,934	30,077	15,302	14,792
Amortization of net actuarial loss (gain)	(2,201)	(1,727)	(10,294)	(6,291)	(1,360)
Defined Benefit Pension Plans [Member] \| Canadian plans [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of the year			123,600	112,672
Fair value of plan assets at end of the year			113,814	99,103
Funded status of the plans			(9,786)	(13,569)
Service cost			1,470	1,112
Interest cost			5,713	5,491
Expected return on plan assets			6,912	5,988
Curtailment			185	0
Termination benefit cost			933	0
Company contributions			4,629	1,698
Participant contributions			498	410
Benefits paid			(8,595)	(8,238)
Actual return on plan assets			10,419	15,649
Net periodic benefit cost			6,231	2,746
Amortization of net actuarial loss (gain)			4,836	2,116
Other Postretirement Benefits [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of the year			59,789	45,592	38,182
Fair value of plan assets at end of the year			0	0	0
Funded status of the plans			(59,789)	(45,592)
Service cost	525	405	1,620	1,525	1,892
Interest cost	780	690	2,775	2,607	2,540
Expected return on plan assets	0	0	0	0	0
Curtailment	0	0	0	0	0
Termination benefit cost	0	2,413	2,413	0	0
Company contributions			2,576	1,572
Participant contributions			1,077	988
Benefits paid			(3,674)	(2,577)
Actual return on plan assets			0	0
Net periodic benefit cost	1,180	3,252	5,783	2,600	3,213
Amortization of net actuarial loss (gain)	25	134	536	1,043	730
Other Postretirement Benefits [Member] \| Canadian plans [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of the year			12,898	11,586
Fair value of plan assets at end of the year			0	0
Funded status of the plans			(12,898)	(11,586)
Service cost			34	62
Interest cost			596	632
Expected return on plan assets			0	0
Curtailment			0	0
Termination benefit cost			0	0
Company contributions			771	665
Participant contributions			0	0
Benefits paid			(771)	(665)
Actual return on plan assets			0	0
Net periodic benefit cost			590	694
Amortization of net actuarial loss (gain)			$ (39)	$ 0

Company's Canadian pension and other postretirement benefit plans.

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The amount of net gain or loss recognized in net periodic benefit cost as a result of special termination benefits offered to and accepted by employees in connection with their termination of employment that are only offered for a short period of time.

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The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

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The amount of gains or losses recognized in net periodic benefit cost

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1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. The projected benefit obligation is measured using assumptions as to future compensation levels if the pension benefit formula is based on those future compensation levels (pay-related, final-pay, final-average-pay, or career-average-pay plans). For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. Prior to an employee's full eligibility date, the accumulated postretirement benefit obligation as of a particular date for an employee is the portion of the expected postretirement benefit obligation attributed to that employee's service rendered to that date; on and after the full eligibility date, the accumulated and expected postretirement benefit obligations for an employee are the same.

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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- Definition

The amount of contributions made by the employer.

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-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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The amount of contributions made by plan participants. Under a contributory plan active employees or retirees contribute part of the cost. In some contributory plans, those wishing to be covered must contribute; in others, the contributions result in increased benefits.

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 106

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those nonbenefit obligations may be considered as reductions of plan assets.

+ References

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

+ References

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References

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The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments).

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The amount of net gain or loss recognized in net periodic benefit cost as a result of an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 88

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 106

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-Publisher FASB

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-Number 87

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

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