Retirement Plans - Obligation and Asset Data for Defined Benefit Pension Plan and Postretirement Health Benefit Plan (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
May 31, 2013	May 31, 2012	May 31, 2011
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Curtailment	$ (2,228)
Riverside Defined Benefit Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at end of year	47,174	40,028
Riverside Defined Benefit Plans \| Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	66,121	57,756
Service cost	339	537	502
Interest cost	2,613	3,040	2,951
Benefits paid	(3,511)	(3,102)
Actuarial loss (gain)	(945)	7,890
Benefit obligation at end of year	62,389	66,121	57,756
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	40,028	39,752
Actual return on plan assets	6,353	(864)
Employer contributions	4,304	4,242
Benefits paid	(3,511)	(3,102)
Fair value of plan assets at end of year	47,174	40,028	39,752
Funded status at end of year	(15,215)	(26,093)
Assumed discount rate	4.50%	4.35%
Average long-term pay progression	0.00%	3.00%
Riverside Defined Benefit Plans \| Health Benefit
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	8,168	7,857
Service cost	106	98	114
Interest cost	352	415	425
Participant contributions	168	184
Benefits paid	(410)	(405)
Actuarial loss (gain)	(2,091)	19
Benefit obligation at end of year	6,293	8,168	7,857
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	241	221
Benefits paid	(241)	(221)
Funded status at end of year	$ (6,293)	$ (8,168)
Assumed discount rate	4.55%	4.35%

Defined Benefit Plan, Benefit Obligation, Benefits Paid

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- Definition

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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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)

-URI http://asc.fasb.org/extlink&oid=6504192

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-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)

-URI http://asc.fasb.org/extlink&oid=6504226

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-Number 132R

-Paragraph 5

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- Definition

Amount of gain (loss) from a decision to temporarily deviate from the substantive plan, or from a change in benefit obligation or plan asset value from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover or salary scale.

+ References

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-Glossary Gain or Loss

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- Definition

The interest rate used to adjust for the time value of money for the plan.

+ References

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- Definition

Expected rate of compensation increases (for pay-related plans).

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- Definition

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. 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.

+ References

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- Definition

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. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

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-Glossary Postretirement Benefits

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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The amount of contributions made by plan participants. This item represents a periodic increase to the plan obligation and an increase to plan assets.

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- Definition

The amount of decrease that is related to 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.

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- Definition

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 non-benefit obligations may be considered as reductions of plan assets.

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- Definition

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.

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- Definition

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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-Glossary Service Cost (Component of Net Periodic Pension Cost)

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