Retirement Plans - Actual Defined Benefit Plan Asset Allocation and Target Allocation (Details)	12 Months Ended
	May 31, 2013	May 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Actual and target plan asset allocation	100.00%	100.00%
Riverside Defined Benefit Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual and target plan asset allocation	61.00%	59.00%
Riverside Defined Benefit Plans | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual and target plan asset allocation	39.00%	41.00%
Target 2014
Defined Benefit Plan Disclosure [Line Items]
Actual and target plan asset allocation	100.00%
Target 2014 | Riverside Defined Benefit Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual and target plan asset allocation	60.00%
Target 2014 | Riverside Defined Benefit Plans | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual and target plan asset allocation	40.00%