Note 8 - Employee Post-employment Benefits - Allocation of Retirement Plan's Assets Held in Trust (Details)	Jun. 06, 2017	May 31, 2016
Actual Allocation	100.00%	100.00%
Equity Securities [Member]
Target Allocation	56.00%
Actual Allocation	63.00%	61.00%
Equity Securities [Member] | Minimum [Member]
Target Allocation	41.00%	41.00%
Equity Securities [Member] | Maximum [Member]
Target Allocation	71.00%	71.00%
Fixed Income Securities [Member]
Target Allocation	28.00%
Actual Allocation	25.00%	20.00%
Fixed Income Securities [Member] | Minimum [Member]
Target Allocation	13.00%	13.00%
Fixed Income Securities [Member] | Maximum [Member]
Target Allocation	43.00%	43.00%
Public Real Estate Investment Trusts [Member]
Actual Allocation	0.00%	5.00%
Public Real Estate Investment Trusts [Member] | Minimum [Member]
Target Allocation	0.00%	0.00%
Public Real Estate Investment Trusts [Member] | Maximum [Member]
Target Allocation	10.00%	10.00%
Cash and Cash Equivalents [Member]
Actual Allocation	1.00%	4.00%
Cash and Cash Equivalents [Member] | Minimum [Member]
Target Allocation	0.00%	0.00%
Cash and Cash Equivalents [Member] | Maximum [Member]
Target Allocation	20.00%	20.00%
Other Retirement Plan Assets [Member]
Actual Allocation	11.00%	10.00%
Other Retirement Plan Assets [Member] | Minimum [Member]
Target Allocation	0.00%	0.00%
Other Retirement Plan Assets [Member] | Maximum [Member]
Target Allocation	21.00%	21.00%