Note 8 - Employee Benefit Plans - Investment Policy Allocation (Details)	Dec. 31, 2017	[1]
Large Cap US Equity [Member]
Investment policy allocation	17.00%
Large Cap US Equity [Member] | Maximum [Member]
Investment policy allocation	20.00%
Small Cap US Equity [Member]
Investment policy allocation	8.00%
Small Cap US Equity [Member] | Maximum [Member]
Investment policy allocation	10.00%
Foreign Government Debt Securities [Member]
Investment policy allocation	25.00%
Foreign Government Debt Securities [Member] | Maximum [Member]
Investment policy allocation	30.00%
Fixed Income Funds [Member]
Investment policy allocation	20.00%
Fixed Income Funds [Member] | Maximum [Member]
Investment policy allocation	25.00%
Real Estate [Member]
Investment policy allocation	15.00%
Real Estate [Member] | Maximum [Member]
Investment policy allocation	18.00%
Absolute Return [Member]
Investment policy allocation	5.00%
Absolute Return [Member] | Maximum [Member]
Investment policy allocation	7.00%
Real Return [Member]
Investment policy allocation	10.00%
Real Return [Member] | Maximum [Member]
Investment policy allocation	13.00%

[1] The policy as of December 31, 2016 does not include minimum investment thresholds.