Note 7 - Employee Benefits Plans - Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Plan [Member]
Interest cost	$ 148	$ 160
Amortization of net loss	84	72
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	232	232
Net loss	138	210
Amortization of net loss	(84)	(72)
Total recognized in other comprehensive loss	54	138
Total recognized in net periodic benefit cost and other comprehensive loss	286	370
Amortization of net loss (gain)	84	72
Net periodic benefit cost	232	232
Supplemental Employee Retirement Plan [Member]
Interest cost	64	68
Amortization of net loss	33	32
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	97	100
Net loss	68	14
Amortization of net loss	(33)	(32)
Total recognized in other comprehensive loss	35	(18)
Total recognized in net periodic benefit cost and other comprehensive loss	132	82
Amortization of net loss (gain)	33	32
Net periodic benefit cost	97	100
Other Postretirement Benefits Plan [Member]
Interest cost	23	23
Amortization of net loss	(10)	(17)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	12	6
Net loss	61	(51)
Amortization of net loss	10	17
Total recognized in other comprehensive loss	71	(34)
Total recognized in net periodic benefit cost and other comprehensive loss	83	(28)
Amortization of net loss (gain)	(10)	(17)
Net periodic benefit cost	$ 12	$ 6