Note 8 - Employee Post-employment Benefits - Allocation of Retirement Plan's Assets Held in Trust (Details)	12 Months Ended
	May 31, 2016
Minimum [Member] | Equity Securities [Member]
Target Allocation	4171.00%
Minimum [Member] | Fixed Income Securities [Member]
Target Allocation	1343.00%
Minimum [Member] | Public Real Estate Investment Trusts [Member]
Target Allocation	10.00%
Minimum [Member] | Cash and Cash Equivalents [Member]
Target Allocation	20.00%
Minimum [Member] | Other Retirement Plan Assets [Member]
Target Allocation	21.00%
Maximum [Member] | Equity Securities [Member]
Target Allocation	3969.00%
Maximum [Member] | Fixed Income Securities [Member]
Target Allocation	1242.00%
Maximum [Member] | Public Real Estate Investment Trusts [Member]
Target Allocation	10.00%
Maximum [Member] | Cash and Cash Equivalents [Member]
Target Allocation	20.00%
Maximum [Member] | Other Retirement Plan Assets [Member]
Target Allocation	24.00%
Equity Securities [Member]
Target Allocation	56.00%
Actual Allocation	58.00%
Fixed Income Securities [Member]
Target Allocation	28.00%
Actual Allocation	23.00%
Public Real Estate Investment Trusts [Member]
Actual Allocation	5.00%
Cash and Cash Equivalents [Member]
Actual Allocation	1.00%
Other Retirement Plan Assets [Member]
Actual Allocation	13.00%
Actual Allocation	100.00%