Retirement Plans - Additional Information (Detail) (USD $)	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Cumulative postretirement benefit plan adjustment, net of tax	$ 24,500,000	$ 12,800,000
Cumulative postretirement benefit plan adjustment, tax	6,000,000	7,400,000
Financial Security Defined Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan expected future benefit payments in 2013	3,300,000
Defined benefit plan expected future benefit payments in 2014	3,500,000
Defined benefit plan expected future benefit payments in 2015	3,900,000
Defined benefit plan expected future benefit payments in 2016	4,400,000
Defined benefit plan expected future benefit payments in 2017	4,500,000
Defined benefit plan expected future benefit payments thereafter	21,600,000
Weighted Average Discount Rate Percent	4.30%	5.15%
Recognized actuarial loss adjustment			4,441,000
Pension expense charged to costs and expenses	9,030,000	6,871,000	12,257,000
Financial Security Defined Benefit Plans | Years prior to 2010
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial loss adjustment			3,400,000
Financial Security Defined Benefit Plans | Fiscal Year 2010
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial loss adjustment			1,000,000
Financial Security Defined Benefit Plans | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit period for participants			15 years
Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Approximate number of employees and retirees of subsidiary	600
Pension and other benefit obligation	88,500,000	74,000,000
Pension and other benefit obligation, current	3,700,000	3,100,000
Riverside Defined Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Contribution expected to be made in 2013	4,600,000
Accumulated Pension Plan Benefit Obligation	64,000,000	56,100,000
Assumed long-term rate of return on pension plan assets	7.60%
Assumed health care cost for the next fiscal year	9.00%
Year for ultimate health care cost	2017
Increase for the health benefit obligation by one percentage point amount	377,000
Decrease for the health benefit obligation by one percentage point amount	388,000
Increase in health plan expense by one percentage point amount	26,000
Decrease in health plan expense by one percentage point amount	26,000
Riverside Defined Benefit Plans | Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Ultimate trend rate in 2017	5.00%
Riverside Defined Benefit Plans | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocations	60.00%
Riverside Defined Benefit Plans | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocations	40.00%
Riverside Defined Benefit Plans | Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan expected future benefit payments in 2013	3,200,000
Defined benefit plan expected future benefit payments in 2014	3,400,000
Defined benefit plan expected future benefit payments in 2015	3,500,000
Defined benefit plan expected future benefit payments in 2016	3,600,000
Defined benefit plan expected future benefit payments in 2017	3,700,000
Defined benefit plan expected future benefit payments thereafter	20,200,000
Assumed long-term rate of return on pension plan assets	7.60%	8.25%	8.25%
Weighted Average Discount Rate Percent	4.35%	5.35%
Pension expense charged to costs and expenses	2,191,000	2,781,000	3,281,000
Riverside Defined Benefit Plans | Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan expected future benefit payments in 2013	400,000
Defined benefit plan expected future benefit payments in 2014	400,000
Defined benefit plan expected future benefit payments in 2015	400,000
Defined benefit plan expected future benefit payments in 2016	400,000
Defined benefit plan expected future benefit payments in 2017	500,000
Defined benefit plan expected future benefit payments thereafter	2,700,000
Defined Contribution Pension
Defined Benefit Plan Disclosure [Line Items]
Pension expense charged to costs and expenses	$ 1,200,000	$ 2,800,000	$ 2,800,000