Note 8 - Retirement Plans - Allocation of Plan Assets (Details)	Dec. 31, 2017	Dec. 31, 2016
Asset category:
Plan assets, percent	100.00%	100.00%
Insurance General Account [Member]
Asset category:
Plan assets, percent	19.00%	23.00%
Cash and Cash Equivalents [Member]
Asset category:
Plan assets, percent	4.00%	6.00%
Equity Securities [Member]
Asset category:
Plan assets, percent	42.00%	39.00%
Fixed Income Securities [Member]
Asset category:
Plan assets, percent	31.00%	28.00%
Real Estate [Member]
Asset category:
Plan assets, percent	4.00%	4.00%